To socialize with this man and his girlfriend is fine. But a man who wants to hang out with you, while his girlfriend is away, is generally signaling a sexual interest. In answer to what you are wondering: No, generally, men and women can not be "just friends."

He's not trying to sabotage the workplace. He's not crying out for support in his recovery. His relationship has it's troubles and he'ld like to bang you. It's just that simple.

Find yourself a female friend. When you're a young, attractive female, it's often easier to attract males than to attract females into taking an interest in you. Why do you suppose that is? Hmmmm, let's think hard.

You don't have to tell this guy you never want to see him again, but you do need to firmly reject any attempts he makes to seduce you. Don't get all hurt. This is a troubled man with little recovery time under his belt. He is not "working his program," as they say. He needs to do that, and he needs to not be distracted by his attraction to you.