He's a waste of time. Honestly, who wants to be some cheating scumbags bit on the side? Nobody. Nobody deserves that either, but don't expect any remorse because he's probably got none to offer.
You have 2 realistic choices in my view: 1) just cut off contact and leave it at that. 2) message the girlfriend to tell her what her boyfriend is like and then cut off contact. As a woman I'd go for option 2 because I would rather know than not, and you may well be doing her a favour in the long run. Though I can appreciate not wanting to go down that path given he knows where you work etc. Either way, get shot of him totally, he's clearly not a decent person.