confront her

You have to confront your roommate about the theft because she has access to the money. You can't accuse her of taking the money, but you can certainly ask her whether she knows
anything to help you. Ask her whether she had any guests for the duration of time involved. In the process, tell her that you are about to go to police. The amount of money will make it a grand theft so if her friend is involved, the friend will be in big trouble (jail, deportation, etc.etc.). Furthermore inform her that you only want to help your mother recover the money and don't want to get anybody in legal trouble etc. etc. If she took the money, she might use the "friend" as a way out
and return the money to you.

Remember you are doing her a favor by confront her. If she did the crime by opportunity, your confrontation will
help prevent her to become a criminal by habit.