You can run one shim, it will be fine. If anything, you will up the compression just a bit.
HHmm.
Whats it going to hurt if he does it right?? Sure it will run, and sure his compression will increase, but why not do it right just in case something happens?? Do you realize that if he takes your advice and something does happen, he can blame you?? Not that he would, but..... You never know....