I think it really depends how your ex feels about you. If he still likes you, then it could be a nightmare. But if he isn't really that bothered, I don't see the problem.
My current boyfriend is a friend of my ex, and I met him through my ex. We still all meet up and go out as a group. But on the other hand, when I still liked my ex, a few months back, if he'd gone out with one of my friends, I'd have been distraught.
That said, if you really like your ex's mate (not an impression I got from your post, just that he likes you, but then why would you ask if you didn't like him...) then you should go for it anyway I think. To be honest, it isn't really your ex's business.