I used to work with a guy who would flirt with every girl I knew, including my girlfriends and girlfriends of my close friends. Every time there was some kind of returned flirting, he would take that as licence to attempt to convince them to leave their partner (if they had one) and hook up with him. One of them actually almost did it, too. Despite that, he was my superior at the time and I could do little about it. Eventually I cracked when he tried it one time too many and I called him out on it with the assistance of about 20+ girls that he had told (in private) they were "the one" or something like it. He left. He's married now, yet every now and then I seem him around and I'm fairly certain he still tries his luck with certain females he used to prey on.

We retained a professional relationship at all times within the workplace, really. I would encourage doing the same.