Satan

This is unlikely and not substantiated in Qur'an and hadith.

Shaytan's job is to deceive us in this world, not at death. There would be no purpose for him to do that there.

Sometimes, when people are dying, they see their loved ones. This is also mentioned in some of the accounts of Karbala that are recited yearly regarding the shuhada'. Whether this is actually seeing them in the barzakh or some other metaphysical phenomenon, God knows best. But it is unlikley to be Shaytan. 

According to the Qur'an, God has absolute power over all things.

Satan, as a created being, is under God's power and only has some capability to influence or affect people because God has allowed this for the duration of this world or universe.

God has given Satan free will, just as God has given free will to human beings and other jinn. (According to most Muslim exegetes, Satan is one of the jinn, not an angel or fallen angel.) However, this free will is not absolute: while we are responsible for our decisions in the hereafter, and lead ourselves to heaven or hell, we cannot always do everything we want since sometimes the divine decree/destiny affects what happens in our lives. So, we are still under the power of God, even if we have free will in how we respond to matters that are fated.

Therefore, Satan is not an equal contender to God and there is no cosmic battle between good and evil, even if sometimes an individual person may go back and forth towards or against Satan.

(However, the idea of a cosmic battle between good and evil predates Islam, and has hence influenced some cultures, and also seems to be part of the human psyche, so you will sometimes hear Muslims framing things like that. But that is not theologically correct according to the Qur'an.)

Some scriptural references from the Qur'an:

* Satan has no powerful over the faithful who seek refuge with their Lord (16:22).
* On Judgment Day, Satan will tell people, "God made promises that were true, and I made promises to you that I did not keep. I had no authority over you, except that I called you and you obeyed me. So do not blame me; blame yourselves. I cannot aid you now, and you cannot aid me now. I disassociate myself from the way you associated partners with God." (14:22) 
* The story of when Adam is created, God orders Satan to bow to him. Satan refuses, and God expels him from the celestial realms. Satan swears to misguide humanity, so God allows him to do that for the duration of this world or universe, after which time Satan and those who choose to obey him will enter hell. (7:11-7:18, and elsewhere in the Qur'an)
* Satan's power is to whisper and delude. (4:120)
* Satan's plots are weak. (4:76)
* People choose by their own free will to obey (and hence worship, since obedience is considered a form of worship) Satan and are held accountable for it. (36:59-63)

These sections of the Qur'an can be read further at quran.al-islam.org or other websites if one does not have a printed copy of the Qur'an.

However, insofar as Satan's power in this world among humanity is proportional to how many people choose to support, follow, or obey him, one could say that, to some degree, Satan's power depends on people's choices. This is similar to a country which has a president who has full authority to exert his/her will (for instance, a military and police force which will do what he says), but where the people follow what a social influencer says. He may allow this and be the ultimate authority, and could shut down the influencer if he wanted, but, in practice, the social media influencer is asserting some authority due to the people's choice. 

So, from this angle, one could say that sometimes Satan's influence is greater or lesser, depending on where one is, what era one is living in, etc. Certainly there has been a lot of evil in the world in the 20th century, however, it is also quite possible that Satan was not behind this and is simply standing there smiling and laughing as human beings make a mess of things themselves.

Some of this is apparently also due to human choice, especially the move away from the divine and the sacred in some heavily secular ideologies; the Qur'an says: "They forgot God, so God forgot them". (8:67) People can do this with or without Satan.

Still, the Qur'an says: Had it not been for the grace and mercy of God Allah upon you, you would have followed Satan, except a few of you (4:83). So this reiterates the idea that God is the ultimate authority, not Satan.

I also like to think that, despite the evils of the world, most people are ultimately good at heart and want for things to go aright; we are just going through a difficult time.

Hope that helps!

There is no theological reason why it is impossible to see Shaytan, although most people do not see Shaytan on a regular basis. 

However, how does your friend know it is Shaytan, specifically? Is he arriving with an ID card? What about just garden-variety shayateen?

Basically there is no way to prove things which are not objectively measurable. That is, there is no way to prove he is seeing Shaytan and there is no way to prove that he is not. All you can say is that he sees something, and this is what he understands it to be. 

It is said that doing wrong acts leaves an ugly imprint on the soul which becomes manifest in the hereafter. For this reason, some mystics are said to be able to see people's true natures. So perhaps this might also be what he is seeing. 

I might be suspicious of the clause "whenever someone does something wrong according to Islam", because sometimes people do things that are ethically wrong but are not obviously forbidden. For instance, it is allowed to buy a knife. However, if I buy a knife to murder someone, this is very wrong. Still, it would not be obvious from my purchase that I am committing a sin.

Similarly, we do not always know when someone is doing something wrong. For instance, someone might be lying or trying to flatter someone in order to take advantage of them, but we do not always know it. Along the same lines, we don't usually know if someone is secretly breaking their fast or committing other hidden sins.

(Of course if he is seeing Shaytan in these cases, it is a different story.)

Anyway, most people do not require Shaytan to do wrong things as we are quite capable of doing them ourselves without him. 

There is nothing in Islam that connects 666 with the devil. The symbolism between 666 and the devil developed in history among Christians over time. There is a reasonably comprehensive discussion of this on Wikipedia: https://en.wikipedia.org/wiki/666_(number) 

Still, it is unusual if you are seeing that everywhere. You should be sure it is not just observer bias. For instance, a person might see many numbers in one day but pay more attention to that number because it is meaningful and a little scary. If it is really showing up everywhere (for instance, several times a day in random places) and an objective observer would agree with it, that is definitely weird, although it's hard to say what that might mean. 

Anyway, it seems like the underlying root of your question is whether you are just having bad luck of an ordinary kind, or whether there is something metaphysically unhealthy going on (for instance, the Qur'an discusses the evil eye).

Bad luck can indeed just sometimes be ordinary and due to decisions that don't work out (such as an investment or job that doesn't work out), the everyday travails of life (needing expensive dental work), or circumstances out of our control (such as how the coronavirus has affected the global economy).

(This is apart from genuinely bad choices which lead to bad results, but I'll assume that's not your case here.)

Sometimes what appears as bad luck is also a way that God guides us to do something else through fate. For instance, I try as hard as I can, but I can't get a job in a certain city or field, so I get a job in a different city or field, and this is par of my fate and a way of moving me on to something else. So, is it possible you are being guided towards something else? 

And sometimes there are spiritual/metaphysical components, such as the evil eye, although that is less common. Anyway, if that is something you are concerned about, you could discuss it with a religious scholar who has knowledge of these things and who is trustworthy and see if they have any thoughts on your situation. 

Also, don't forget, if you aren't doing so already, there are du'as and surahs you can recite to seek an increase in sustenance from Allah, for instance, Surah al-Waq'iah on Thursday nights. You can read more on duas.org.

Hope things get better for you!