$20 an hour... nice. After taxes they're making around what, roughly $12 an hour? I'd imagine their skills are worth just that; $12 an hour! I'd bet your customer comes crawling back complaining that the these jokers are not nearly as skilled. Which is why you leave the relationship on good solid terms.
And when they do come crawling back, give them this letter and make them sign it before you'll do any work:
Dear <your name>,
I, <customer name>, am very very very very very very sorry for doubting you. I have seen the light and the price you charge is more than reasonable in this industry. The cheap ass designer I switched to was a bucket of crap. You are the best designer in the world. You are the messiah of web design, and they should have talked about you in the Bible like "The father, the son, the holy ghost, and <your name>".