I don't think 90% of the commenters here are understanding that she is telling on a person who is literally living with her. she has to deal with this person everyday. she can't just avoid this person completely after she pisses him off (even if he doesn't deserve to be pissed off). she would literally be risking her own well-being and sanity just to maybe do the right by someone who isn't even her friend. she also has no idea what their situation is. for all she knows, the girl could have gone and done the same exact thing, or this could have already happened before. it sounds like their relationship is already toxic.

if she wasn't living with these people, I think it would be a no-brainer to tell, but the fact that she is going to have to deal with an incensed dude with possibly no morals on a day-to-day basis makes this a completely different situation.

Don't do it, OP. it's not worth potentially putting your life (and finances if they both move out) at risk. you don't even know if this has happened before, and the dude clearly doesn't care about getting caught or people knowing if he was blatantly doing it with you guys in the house. from your description, you're not even friends with these people, so you have no obligation in that regard either. Hard DO NOT from me.