To be honest this thread is mainly meant as a joke, I know I'm not going to let such playing make it to the final product, whatever the cost. It's just that my attitude towards clients is to be as friendly as possible (as long as it doesn't deteriorate the quality of my work substantially) and telling someone the harsh truth is rarely well appreciated by him.
The main problem is that I know for a fact that he has practised a lot because he knew his parts and the song order and the band can testify to that. However, he has very bad picking technique and a mediocre sense for rhythm at best. Some people just don't really have it in them but I can't tell him "ehm...have you tried taking up bowling or something else that doesn't involve desecrating an art? Yet they don't have a deadline so he'd insist on rehearsing more unless I give him a good reason why that would be futile.