I think there's two types of men that are into girl failures:
1.) The type of man who sees an easy woman. She doesn't have her life straight, and he can either take advantage of that, or "fix" her.
2.) The kind of man who's ALSO a failure, and so they'd know how to talk to someone who's similarly down and out. If Fiona was some businesswoman, he'd have no chance with her. But Fiona being a bum is perfect because the man is ALSO a bum, so he'd know exactly what to say and do with her.