There is nothing much you can do if your best friend is dating your ex-boyfriend since you two are no longer together. There is a reason why you separated with him and your ex should always be your ex.
Honestly you're her best friend. And she is yours. You said that he's a cheater. It's your RIGHT to be honest and upfront with her. Tell her the truth. Tell her that he might just be telling her lies to be with him. You knew her since she was 5. That's years of friendship and trust put into each other. If she truly won't listen to you. Be there for her if he does choose to cheat on her and comfort her when she will become sad. But support her, if he chooses to learn from his mistakes on cheating and becomes a kind and caring BF who puts his loyalty to her above anyone else. Protect your best friend, but love what she is having right now and hope that he won't cheat on her. If he shows signs that he will. Tell her, or better yet. Put him in a position where you make your friend hate him. That way the feelings of sadness she'll get wont be nearly as bad, if he were to cheat on her and she finds out all by herself. Tell me anything else you left out, so I can help further.
I'm dealing with this problem right now. I think its best to stay neutral with her but not be as close as you were before. find new friends who actually respect your feelings, find a new guy . . . anything to help forget about how the problem i hurting you. hope this helps :)
if you had a bf and broke up with him one night and he asked her out the next morning and she lied to your face that she doesnt but they do and he wants to be with both of us and he misses me i know i shouldnt broke up with him and now i realize that i really did love him and i dont know what to do
it literally just happened to me today and I find it retarded. My friend pretended to hate him and pretended like they were enemies and then after he admitted to liking her she admitted to liking him and she had a boyfriend and me and my ex were on and off and a couple days ago we were on and instead of hanging out with me he went to hang out with her! They flirted with each other right in my face and she would always ask him for a hug in front of me. Im still like in love with my ex and she knows it. Right now we aren't talking. My other friends and I have this thing where if I still liked someone I broke up with and he asked my friend out they would ask me if I still liked him and if so he would be off limits to them. That's what a true friend does and all the others aren't friends.