My best advice is to put yourself in your friends shoes. Ill use a real life example. I was dealing with a DJ in town, I was in love with him, God rest his soul. He passed recently, I sent my friend a picture of him she said "Omg, thats my friends man". Do I think she told her Idk. Was I mad yes. Was I so blinded by love I stayed with him yes. My friend cherished my friendship also her other friend. It would have been unfair of me to say to her pick a side. I did that too in the moment and almost lost a 20 year friend back then. I would ask you to continue to be her friend its not her fault. When hes long gone she will be there. I dealt with that man from 2013-2019 and did not know he was married to the other girl until he died. Had he told me I would have dropped his ass and he knew it. Thus why he did not. Im sure she didnt know about me either she was gullible like me working crazy ass night shifts. We werent committed just off and on. Moral of the story its HIM not your friend.