How to convince my best friend to leave her partner?

By partner, I mean they are not in a relationship. Long story short, my best friend finally caved and gave this dude a chance who had been crushing on her for a few months. This started on Halloween, and at first I was really rooting for him, but it didn’t take long to change my perspective. One of the biggest issues is that he is stringing her along. After only these few months he tells her he’s in love w/ her (she said it first), but yet won’t officially lock in a relationship. I know it’s because of other women, since multiple people have tried telling her, not to mention I’ve caught him with same two others girls a few times when I go to the bar w/ my boyfriend. Even recently he went out with his ex to dinner, and when my bestie tried to get the truth, he denied it repeatedly before she literally had to tell him that someone saw him on a date, then he came clean. He flirts with others in front of her (more recently he actually sucked/kissed her close friends fingers right in front of her, then called the next day apologizing profusely). I’ve never seen my friend hurt so much over a guy, like she’s cried herself to sleep and had a breakdown at this same bar her frequents bc he was ignoring her to drink with one of the other 2 girls. It’s just constant disrespect from him, and so toxic for her since she already struggles w/ depression and really wants things to work with him since they don’t seem to with anyone else. I try to be removed, but she pulls me back in desperately wanting advice about him, which she never takes. I want to see her in a healthier situation but she doesn’t want to be alone. Is there something I can say to help her leave? Or should I ask her to stop talking to me about him and let her do her thing?
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I want to say: I feel like I’m at a place where if you are going to stay with him then I just need to stay out of it completely and not hear about it. I never want you to feel like you can’t talk to me, but it’s just like the more he hurts you the more resentful I get towards him and I don’t want to feel that way. Especially if he’s someone I’m going to have to interact with often in the future. I think you’d feel this same way if the shoe were on the other foot, so I hope you can understand.
How to convince my best friend to leave her partner?
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