dinghy stories? he has a boat?

Yeah, that's great and all as long as he never finds out that you guys slept together. You'll probably be fine, hell, you can probably fend him off if he comes for you or protect her if you guys are caught together but if he finds out from her when those two are alone then...it'll be more than the usual abuse.

If you're gonna be with her then don't flaunt it dangerously until you're safe. More than that, boyfriend A might be a scumbag but it sounds like all you want is sex. You talk about how good it was and how you'll be "ridden into the sunset". If she's leaving her abusive boyfriend for you it's because she feels safe with you so you better offer her more than just sex, she needs love.