It's simple. Either A. Act like he doesn't exist. Which can be very fun. Or B. stop giving a @#$%. If she's what you want, just tell the piss ant right up front how you feel... All of it. Starting from high school to now. If you guys were friends, wait, real friends, you should be able to talk it over.
In the end, I wouldn't care either way and this wouldn't even be an issue in my mind. I've got the girl, her folks like me, end of story...