My friend staying in a relationship knowing his bf cheated on him

So me 18f and my friend same age as me has boyfriend since we were in the 8th grade and i have hated this relationship since day 1 i knew who she was dating it was our senior in 10th grade also her cousin me and her being a muslim it might be common for musilms and even some parts but i personally hate the idea of dating or marrying a cousin and she knew it but as she was close friend i just wanted her to be happy but that piece of sh*t got ahead and cheated on her and guess who she was it was my friends older sister🫨🫨🫨 i was so freaking out of my mind and seriously cursed them all her sister blamed my friend saying it was her fault that this happened and all this shit wasn't enough she forgave them both and remained in a relationship with him a few of my other friends and seniors told me they saw her cousin kissing a girl i let her know but she wouldn't believe me and ended our friendship and since then she hasn't talked to me