I think what you are really asking, is for us to reveal what would be a very special truth: That your ex still secretly likes you, regrets that the relationship ever ended, and is only going out with the new girl as a surrogate in the hopes of getting you back
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That further, he knows he was wrong before and would give almost anything to be able to correct his errors, but doesn't know how to to talk to you about it.
That would feel so good for any of us to hear about an ex, as long as they weren't a psycho stalker!
Unfortunately, none of us has enough information to make that much of a sweeping statement accurately . . . but hey I would dwell on the possibility for a while just to make myself feel a little warm and cozy
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In reality it seems like he either wanted to talk to you or has some remaining warm feelings for you that could range anywhere between just feeling pretty friendly and well disposed towards you and outright romantic intentions.
Also, it speaks well of him maturity wise that he was willing to take time to talk to someone that he probbaly had pretty significant feelings for at one time. Cause if he just wanted to be friendly he couldn't have been unaware of the potential akwardness . . . and if he is interested romantically, he had to be hurting a bit inside. Most people would have ignored you if either situation were true. Or hi and byed you.
His only real mistake I can see was chatting like that in front of the new girl, even if she was just a friend it was implied that they were escorting eachother to the event. So a little attention her way would have been politer.