Need advice on how to handle this?

So I have a female best friend ( I have fallen in love with her and she knows this) first time I spoke to her on the phone she was crying about her recent ex which was a toxic relationship and constantly on and off and she was finally doing well. Not seeing him every now and then and giving in to weakness and meeting him and sleeping with him. I've been there for her the whole time because I've been in that situation with someone being manipulative to me and emotional abuse etc... Well the other week she had a moment of weakness again and had him come over for a hour and a half... I warned her it wasn't a good idea and she promised nothing would happen she just wanted to get proof he isn't a good guy to show the girl he just started seeing which I did research on and she was also emotionally vulnerable... Well my friend went through with it and I went back over soon as he left and asked if her plan went well... She slept with him and did not go through with her plan... She said she regretted it and that it was completely over and going to block him etc... 2 days later she is now back together with him... Promised me she would take things slow and be careful especially as she has a daughter who is at the age where she will get attached and when it goes wrong again her daughter is going to be heart broken and it will make it a lot worse as kids tend to bring up the ex a lot once they stop being around... Well my friend broke her promise a day later. He has moved in and is around her daughter... I'm scared I'm going to loose my best friend and he will get her to stop talking to me or never see me because that's who he is... I want to be there for her and support her regardless of her choice but it's so difficult because of what he has done and how already a week later I've not seen her once, hardly spoken to me... I don't want her to be hurt again and makes my blood boil thinking about it all... What do I do. ☹️
Need advice on how to handle this?
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