I don't think she's trying to rationalize dating him at all. I think she's already ruled him out completely, and her older cousin is just trying to figure out why "nice guys finish last".
I mean, that's really the situation - you have a young man with (it seems) all the potential in the world, with just some things that need to be worked on, and he's probably going to get passed over for some prettyboy or thug who's skating by on looks/intimidation/etc.