How long were you guys dating? If this was like 3 weeks you're blowing this out of proportion. If it was like a year (even in high school), your friend is not your friend. Even if he doesn't think it's a big deal he knows you do.
It's not a big deal to date a friend's ex but not if the two parties were that serious...unless you think you're going to marry the girl, just find a girl who your friend isn't still broken up about.