[DeadlySin]

I'd ask your friend first.
Just because it didn't work out with him doesn't mean it couldn't work out for you?
I know most people say its bad, but you could be missing out on a chance to get to know someone really great.
I admit it could make the relationship between you and your friend awkward.
Just ask him first if it's okay.
If not, decide whether his friendship or the girl is more important.
If he says yes, just don't get all friendly with his ex girl in front of him or talk about her to him.
Just try to keep it on the DL.
Then it should be okay. =)