I don't want my best friend to remain friends with the man she cheated on her ex with
Are you sure that she cheated on him?
I know this for a fact, I was her best friend and what she did to her ex-boyfriend was unforgivable.
That must be hard for you to deal with. If she's your best friend, she shouldn't be in relationships that make you uncomfortable. Try talking to her about it.
It's not that hard for me to deal with, I just don't want to see or talk to her again. She made her decision.
I am so sorry this is what ruined your friendship. It really seems like a sucky situation, and if you feel the need to break up your friendship I can't complain.