Cut ties with him. Stop entertaining him. Stop whatever it is that you're doing with him.
You are aware that he has a girlfriend now, don't lower yourself by continuously seeing him.. even as a "friend".
Do you want him or something??
Why wanting to contact his girlfriend telling her he's been with you, offering you sex, doing other inappropriate things while you two were "hanging" out being flirtatious and inappropriate?
Stop flirting with him. Stop "threatening" or asking if his girlfriend knows about him seeing you...playfully or otherwise to tell his girlfriend.
You don't know what his genuine reaction will be if that ever were to happen.
You are being inappropriate by continuing to communicate and perhaps see him.
I would stay out of it. She may know already what type of man he is, and tolerates it.
Just because someone is marveling, gushing, and gloating over a relationship and partner. ..doesn't mean that they are happy with their relationship and doesn't know about their partner being a slimeball.
Also, you don't know what type of person she is.
Leave him alone, and stay out of it.
It would be different if you were a stranger or close to(knew of her) and saw him with being inappropriate.. if that happened, if you had proof..
I would maybe suggest send her it anonymously ...