My best friend cheated on my ex's best friend- and now i'm wary of her betraying my trust too?

Er, why do you care? Who cares! If your friend is happy then leave her be~! If she comes crying to you just be a friend to her. As for the other guy, he sounds wierd needing his friends so that his relationship will work out. WHAT A BABY! Sounds like she made the right choice!