^^^this.
I was just in a situation similar to this, ie. she was already seeing a guy when I first started seeing her. Being more fun and interesting than him is vital. Make sure on your date you do something fun and a bit different so you really stand out from the other guys. Act carefree as to whether she keeps seeing you or not as you have plenty of other potential girls who would love to be with you. Do not be clingy or needy at all.
but yes he sounds like a freak for flying to see her for a few days but never diss him in front of her
good luck.