Roommates new gf is cheating!?

okay so here is a crazy story and i need to know if i should tell her bf? okay so my roommate hooked up with a girl from work who has a boyfriend. my roommate takes her to vegas and her bf (whom she lives with)thinks she is in Cali with her mom....well she was at my house last night and cuddling with my roommate. She tells my roommate that her and richard are pretty much done. Well she texts and texts all night last night and I knew it was her bf she was texting. My roommate asks her if he can read the texts, usually she lets him. this time she says no please dont' read them. Well he never did and she left her phone on the counter last night and this morning i checked to see what she was saying and she was texting her bf saying i miss you i can't wait to cuddle with you when i get back from cali and all this other bs. I am so tempted even though I dont' know her bf to text him from a friends phone and tell him where she has been. he seems like a good guy or do I just let karma takeover