My friend is cheating...

So she is 16 years old and I found out that she has been cheating on her boyfriend with a girl. Now the boyfriend was openly aware that she was bi, but it wasn't a open relationship. The girl she was dating doesn't know either but I have no way of contacting her. So my question is should I tell the boyfriend and if I do what should I say to make it a easy as possible for him to process. ( Also I have told my friend that cheating was wrong but she is acting like a brat frankly telling me that I am a problem in her life...Help!) Thanks!