Why discourage him? There’s absolutely nothing to lose when doing an appeal, save for several hours that he would spend working on it. Just because UCSD is better doesn’t mean it will fit him; he should appeal to where he wants to go (and maybe visit afterwards).
The guidelines on how to appeal are unclear, but I don’t believe just fixing what you messed up on would do. Try and provide them with some new, appealing (haha, punny) info!
Good luck.