You don't. I dated one for a short period before I'd ever heard of such a thing and the best and safest thing to do is leave, run and cut all contact.

They're incredible at lying. It's second nature. Like breathing. It's such a part of them I doubt many even know or are fully aware they're doing it. Or at least acknowledge it long enough to change. Perhaps they're incapable and last I read up on it, most are beyond help even if they wanted it. It's as if a part of them, some natural survival instinct that's been wired wrong is always "on", always patient and waiting and listening for the first chance to spin something. It can even be something innocent for attention but one lie will soon spin into another, tangle up with more and spiral out of control and you don't want to be caught up in that. It can be destructive.

I got out soon after it was pointed out to me what she was and I was amazed how I'd fallen for her shit so many times. Others I know haven't been so lucky and in one case it's led to a baby that would've been mistreated or even abandoned had it not been for the kids' grandparents.

Don't deal with them. Run.