Marrying a friend for citizenship?

A friend and I got into a deep convo recently about marrying for citizenship. A close friend of his was telling him that a close friend of his that he is moving in with asked him to marry her. She is from europe, and would like to be an american citizen. They have been friends for a year + and it would be strictly platonic. He is moving in with her anyway as her roommate. Would it be easy enough to do, considering they were friends already, and now roommates? Is it do-able?