I'm not sure about this specific situation, maybe there's some rule concerning people specifically that I'm not aware of (there's a lot of things you avoid saying for politeness sake so who knows), but using a negative na-adjective to modify a noun is not wrong. Or maybe they tend to prefer using positive adjectives when possible. Not quite sure.
好きじゃない人 - (Person I don't like) seems to get quite a few hits on Google (I'm married to someone I don't like, what do I do!! - or a similar context haha).