She'll prob think I'm a bitch of the highest order for tempting her perfect boyfriend.
Unlikely. You'd have to have a very low opinion of women to think they are all that stupid. I've never met a woman who thinks her shagger bloke was perfect and it was all the OW's fault. But yes, she'll have a very low opinion of you which is understandable, because your behaviour towards a woman who's just given birth was really bad.
It sounds like he was playing you, really. He probably knew you wouldn't let him move in and so wanted to pass the blame for him not leaving on to you, even though he had no intention of leaving his family.
But honestly, you aren't proposing to dump this on her out of any concern for her future. Really you aren't. Nothing's got worse for her since you ended it. Her immediate future was even more threatened while you were shagging her bloke - and you weren't concerned for her then.
You'd be doing this to punish him for not loving you enough to leave. I'd guess that stings and hurts right now. That even a crap man doesn't want you. If you've had a bad or non-existent relationship with your dad, this might really fuck you up for a while.
Go and see a counsellor to help with that and to find out why you got into this in the first place. Then you'll learn to swerve relationships and men like this in the future.