I'd date him too.
The only time I wouldn't date someone is if they were a friend of mine's ex. My girlfriend's ex husbands and boyfriends, that is, would be off limits to me. I've seen it happen before and I've seen friendships end over it, and I don't think any man is worth losing one of my good girlfriends over.
But this is your ex's friend, not yours, so I say go for it.