If I may... it sounds like pretty typical behaviour for a gal who's romantically attracted to a guy she knows is unavailable. She will pretend to ignore him in public or keep him at a cold distance as way to create 'public deception' and possibly also to prevent him from falling for her or getting the wrong idea. This is probably just her way of showing the guy that she respects that he is in a relationship with someone else and has no interest in compromising it.
Just chatted to the guy and it appears the above is the issue. She digs him and out of respect is holding back.
Only problem now is that the guy now sees this as a challenge. Don't know whether too laugh or cry for the dude