I don't really like it

I had lunch with Dany and we talked. You know the usual: “how's business”, “how's the kids”, “do you still work with…” About halfway through the lunch, Dany pulled a printout. It's was an email from a customer and here's just the interesting part.

…
I don't really like it, so I won't be paying you. Can you send me the original files and my nephew will finish it.
…

Dany's a very nice guy. He does some web development, some photography, some design… He's pretty much a jack of all trades. You need it, he can do it, get it or arrange it. I'm not smart enough to do all of that.

…
Me — Did you get a deposit?
Dany — Yes but it was only 10%.
Me — Did he approve your design?
Dany — Of course!
Me — Did you guarantee it?
Dany — Yes, I did.
Me — Me, I'd send him a message that:
1. He approved it, so he liked it then.
2. If he doesn't like it anymore, what specifically doesn't he like. and I'd wouldn't mention anything yet about the nephew, the situation may be salvageable. Most likely he'd rather pay his nephew than you and his nephew would be lot cheaper, especially that now you have done the hard work.
Dany — I'll call him in the afternoon.
…

It's so much easier to give advice when you are not involved.

My own take is that if the guy wants the files so he can turn them over to somebody else, I would charge for the files, about 75% of the total cost of what was already agreed.