In general yes. Although, it may obviously change if there is a piece of clothing that one considers to be unattractive, then she won't look as nice. I mean, she'll still look cute, I'm just trying to say that it doesn't make every thing she wears cute as well.
Well, clothes can help, and it's not a good idea to go out of your way to wear stuff that makes you look worse. For the most part though I'd say yes, if he is attracted to her she will look good in anything. He's probably been thinking about what she looks like in nothing anyway. ;)