What would you do?

[loserguy]

People usually break up for a reason and that very reason usually prevents them from getting back together. The very fact that they were together in the first place would suggest that there may still be some feelings and those feeling may only surface afterwards. Then again everything may be cool. As they are no longer together they should have moved on.

If you like this guy, then go for it. He is not off limits.
By all means tell your friend but do not allow him influence you. It is up to you to find out about this guy for yourself. At the end of the day it is none of your friend's business whom you date and if he has a problem with it then he will simply have to deal with it!