Depends on how much crazy you can take …
Depends on how much crazy you can take.
Because the operative word you used to describe her is “crazy,” I’d say there’s a fair chance she’d retaliate in a way that lives up to that adjective. Instead of reacting like a sane person and dumping her piece of poop boyfriend for doing this to her, she's liable pull some thoroughly unpredictable shit. This could range from slamming you on social media, to harassing you at work, to beating your face in, to putting a severed head in your bed. Who knows! She’s crazy.
I’d honestly hold off on him until they’re done having a shitty relationship that cockblocks you at every turn. Like it or not, this dude still has a girlfriend even though he doesn’t want to. It doesn’t sound like he has feelings for her anymore, but that doesn’t give him license to fuck around on her without consequences. He has to very clearly end it with her, or negotiate some sort of open relationship if he wants to go about tapping your ass in the PC way. Otherwise, if he doesn't make some more definitive moves to end it with her, you’re just the girl he’s cheating with because he’s too much of a testicle to leave this chick. You're not his partner or girlfriend or anal sex friend until he's available to be the same for you.
On the other hand, there are a billion non-PC ways you could go about this if you think you can handle her. Maybe you're a psycho hose beast in your own right. In that case, you could just launch right into this relationship with him and hope that doing so sends the message to her that it's over. She might bite you and draw blood, but if this dude is worth the little interpersonal war you're waging, do what you need to make yourself happy. Just take responsibility for your role in influencing the outcome of whatever shit stain of a relationship they have left.
Decide if you want to bring that drama on yourself right now, and if that guy is worth the months of weirdness, awkwardness and pain that’ll ensue after he tries to extricate himself from his relationship to be with and/or fuck you. I can promise that with this crazy-clit person around, it’s not gonna go smoothly.