It's perfectly natural for you to be a bit cautious, especially considering that in 10 years of being divorced you havent dated much which tells me you got pretty badly hurt in your last experience.  I've got several exes that I'm still very good friends with, and all of them live within a 20 minute drive of me...my girlfriend knows about all of them, and we all hang out regularly.  If this guy and his ex are friends, i don't think it's much to worry about.  I know it's rare for a guy to stay friends with an ex, but it does happen, and if you like the guy and trust him when he tells you she's just a friend I'd let it go til you know otherwise.