If I find out that an adult friend of mine is getting into some major sins, such as homosexuality, and her family does not know about it, should I tell the family to take care of the situation or keep it a secret?
You must do what ever you can to help your friend and protect her from falling in dangerous sinful acts. You must be completely sure that she is doing or going to do that sinful act. You can inform one of her family members who has more influence on her but not every one if you think that that member of her family can help saving her from sinful acts.
Wassalam.