He’s Cheating

My boyfriend’s best friend has been dating someone from my school for about a month now. A couple of days ago, the guy was bragging to a group of us about how he had sex with his ex girlfriend last week while dating this other girl. He had no remorse whatsoever. He then proceeded to try hooking up with my friends. I’ve hung out with the girl a few times and she’s so lovely, but we aren’t REALLY close, but I feel so guilty about the whole ordeal. I would want to know if my boyfriend was cheating on me, but not sure if I should tell her - especially if it would cause a rift in my own relationship. Advice?

UPDATE: I made an anonymous Instagram account and dm’ed her. I saw her on the train looking a little mellow and showing her friends. I peeked at her screen and saw that she was sending a lot of messages to someone. Not her, the guy or my bf have messaged me about it so who knows what’s happening. I feel good that I told her, however she may not take it seriously