I'd be "Gee, you may be right. How about you go be right about me on the street? As a shitty person, I don't feel a need to give charity to ungrateful creepy stalkers."
He does sort of have a point---people shouldn't get pets they don't know how to take care of, and if you're a fish-fancier type which I'm betting he is, that's gonna be a pet peeve. But it isn't necessarily obvious just what you need to know, and she wouldn't have bought the damn fish in the first place if he hadn't scared her to death. But above all, she didn't ask to have him freeloading on her and clearly doesn't much want it and yet she has given him a roof over his head so he can just bloody zip it. He knew there was a problem the day before, but did he offer to help? No, he just made a snide remark, useful as tits on a bull.
This is one of those things--I know the whole point of this is that he's going to have some evolution as a human being until he isn't like that any more and he's supposed to be some kind of diamond in the rough and all that shit. All I can say is it's gonna have to be awful convincing. If it isn't I'm gonna drop this like a hot turd.