We once had a play that was called wrong by the umpire. We argued the application of the rule to no avail. While we were sitting around waiting for our next game he came up to us and told us we were right after looking it up. While we would have protested it in bracket it was pool so it wasn't that big of a deal but I thought it was great for him to do so. I think your friend will do fine with that attitude.Nothing you really can do. Know when to pick your battles, in a regular rec game, let it be, but if you are in your rec playoffs, then make a stink.
A friend of mine just started umpiring. His 2nd game ever the pitcher wound up to throw but didn't release the ball, illegal pitch, ball. My friend, never seeing that before, called it a no pitch. The coach comes out and said he was wrong. My friend calmly told him " coach, this is my 2nd game ever, I never saw that before so I called it no pitch. You may be right, but I don't know that so my call is going to stand and when I go home I'll look it up. " I thought he handled that perfect.