Secretly cheating with long term boyfriends little brother

My friend has been with her boyfriend for 5 years. He treats her really good she moved in with him and his brother. The only complaint she has ever told me is the sex it really boring and not good with him.

Then she told me that one night when her boyfriend went to bed early her and the brother were drinking and ended up having sex. I was in shock when she told me I never expected her to do that. (She has been my best friend for 15 years and she was never the type to cheat). I asked her if it was only the one time and she admitted they have had sex about 10 times since then always when her boyfriend is at work or falls asleep early. She says she doesn’t love the brother it’s just sex nothing else. I feel really bad for her boyfriend he doesn’t deserve that it’s bad enough she is doing that but how could his own brother do that to him. I get she is 100% at fault for this too but the brother is such a smug asshole who doesn’t give a fuck about anyone but himself. My friends boyfriend and his little brother are like night and day differences ones sweet and kind guy and the other is the most selfish guy I’ve ever met. Her boyfriend even let him live in his house rent free because he couldn’t hold a job for the life of him and the parents kicked him out. Is it terrible I want the boyfriend to find out mainly because the brother shouldn’t be betraying his trust like that? Don’t get me wrong my friend needs a wake up call of reality too because she is in the wrong also. What would you do in this situation?