Most Helpful Guy
Are you all okay with that?
Most Helpful Girl
To me it's only wrong if the relationship ended because your friend was betrayed or cheated on or otherwise wronged by the ex. Considering he didn't do anything wrong to her, and it was mainly just because they didn't get along well, I see no problem with it.
BUT don't be sneaky about it or try to hide it from her. Maybe even talk it over with her beforehand. If she's a reasonable person she shouldn't flip out or act like you dating him is a total betrayal or some melodramatic crap like that. If she IS immature like that then, well, whatever. It literally makes zero sense to let the feelings of an unreasonable person influence or control you or your decisions in any way.