I NEED HELP😭😭😭😭😭😭

I went to a family dinner with my boyfriend and I was sitting down with his cousin and girlfriend but the girl left to take her dog out 😑😑then her boyfriend which is my boyfriend cousin 😥😥😥😰😰told him he cheated on her 😑😑😑I got really mad because my boyfriend told me later that his cousin cheated on his girl in the same apartment while his girl was asleep (ate the girl out then had sex with her in the bathroom)👿👿👿👿👿😑😑😑😑😑😑😑😑😑😑😑btw I am not friend with this girl but I feel like I need to tell her 😓😓😓what should I do???