Best friend’s BF is cheating

My best friend is in an extremely toxic relationship. She’s been obsessed with her BF of 18 months and can’t leave his side or she has a breakdown and he’s extremely controlling. She doesn’t speak to anyone but him and barely her mother. I practically have to pry communication out of her. Her boyfriend recently contacted me and started flirting with me, hinting at sending pics and saying he’d be a better match for me than my boyfriend. I have some screenshots and minimal evidence. I’m afraid if I show her she’ll either push me away and be in denial or shut down and be in a disastrous mental state. Any advice would be great because I just want her to be happy. He’s thinking about dumping her anyways apparently because he doesn’t wanna do long distance and he’s moving a few hours away. Should I just wait to see if he does break up with her when he moves? Do I tell her immediately and deal with whatever she does, whether it’s breaking the friendship with me or having a crisis? I’ve been crying about this for days and I’m so stressed and confused. Please be kind, this is a hard time for me although it may seem simple. There’s a lot of personal complications.