<human>: Condense the text into a few key points

I don't want to include too many personal identifiers on here, but here's the synopsis: 

I knowingly have observed my roommate (of a year or so) hanging out with another girl (other than his girlfriend, who has recently moved in with us) for a while. She is foreign and comes from a lot of money-- she has bought him gifts without him telling his actual gf where they have come from. 

My roommate has A LOT of loans and has historically made a lot of decisions that have been spiteful toward me, irresponsible, and cost me a lot of money. At first I thought he was okay and had good intentions, but my roommate is a textbook narcissist that is incapable of thinking about how his actions affect others (it's been difficult, but the rent is so cheap I really can't afford to move). 

Well, he recently has informed me that he is going to marry the foreign girl for $40,000 from her father. His actual girlfriend is aware of this fact and supports it since he is in so much debt. It's really an unhealthy situation for me, but whether I want to condemn him to prison is a whole different story (I really don't). On the other hand, what are the legal obligations of my position? Now that I know... I feel that if I get questioned by somebody investigating the marriage, I don't have any deniability since he has told me. 

My roommate is known  for lying, so at first I thought it might be one of those situations where he exaggerates things. Additionally, it's a good excuse for him to continue contact with this other girl without his girlfriend's permission. However, his girlfriend told me the other day he was out getting "engagement photos." 

I know, I know I should keep my mouth shut... but honestly after him ruining a lot of things with his irresponsibility, I don't know what to do. Also, how does the government go about proving fraud, anyway?
<bot>: Should I turn in my roommate for marriage fraud? He's not my favorite person, but I'm afraid this could come back to me and I'm not sure I want to send a person to prison.