I would add that "October" was only 1-1/2 weeks ago. I don't know when the girl contacted you that he was dating her but. . . That doesn't sound too stable. What bothers me is that you don't have anyone you can "trust" since he's done this before and since the girl telling you, she obviously has her own interests at heart and not yours so it's not like either of them are likely to tell you the truth?

The whole mess doesn't not sound promising. I'd move on, as painful as it sounds like it would be.