Ethically, intentions behind a marriage can be a grey area. Shari'ah itself does not specify what emotions one has to have to conduct a shari'ah marriage contract.

However, what you are saying is often a bad idea.

First, it might have repercussions on your own life. If you don't wish to be with this person, you will be putting your life on hold for their sake. You might end up developing feelings for this person but they do not develop reciprocal feelings, and this causes hurt. This is quite common, especially if you are sacrificing for them (and hence investing your life in them) and if there is a shari'ah bond.

It is not uncommon for the man in this situation to have another wife or woman in another country or on the side who is waiting for him to get the residency, and who he plans to dump the visa-wife for after he gets the residency. This, in the end, can cause the woman to feel betrayed or hurt, even if it is already known.

If it ends up being a sham marriage (that is, you don't actually cohabit), that can cause legal problems or imprisonment in some places, and there is the shari'ah sin of lying on paperwork, etc.

If it isn't a sham marriage, what if you end up with children, financial entanglements, or other things?

Ask yourself, would this person do the same for you? That is, would they put their life, and any other relationships, on hold for you so that you could get residency somewhere? If the answer is no, definitely rethink it.

Often, these situations are associated with secondary sins, such as lying, fraud, or deception, and those are a shari'ah problem and a sin. (Further sins might be - assuming that you do a shari'ah marriage - looking for another partner while you are married, neglecting your spouse, and that sort of thing, since a shari'ah marriage does bring on some ethical responsibility.)

If you have an interest in getting married and settling down, and especially if you are in a younger age bracket, it is better to focus on finding a spouse that you want to be with, since the Islamic ideal is that one gets married and is settled; this should be the priority. If this person really cares about you, they will agree that your future and well-being is more important than theirs.

It is good that you want to help this person and it would be better to find other ways to try to help them, such as pointing them to a good lawyer, or even helping them find someone to marry in a real marriage for their residency.