If he were to continue interacting with this female co-worker in a flirtatious/sexualized way, what would you be prepared to do? Suppose you came to know that he was pursuing this young woman out of sexual interest, and suppose he rejected your advice to end it, would you feel obligated to take some action?

If it seemed like he was returning to his womanizing ways, how would that affect your willingness to be friends with him? Do you think there could come a point where you would advise his wife about his inappropriate behavior? Do you have a friendship with her also?

Have you considered the possibility that his womanizing tendencies might cause him to someday come on to you? People do have a tendency to relapse into old behaviors. How do you think you would handle it, if he were to make advances towards you?

Finally, is this something that, at some point, if it continued, you would feel should be brought to the attention of your employer? Could it impair the ability of co-workers to function and carry on their responsibilities?