First off kudos for trying to help educate him & steer him in the right direction when it comes to caring for his snakes.
The thing is, he's not entirely wrong either. Yep some garter species will sometimes opportunistically take mice in the wild, but I don't know of any that take them as the bulk of their diet. I also am of the belief that a strictly rodent diet isn't necessarily the best for them either. People have shown it can work and it's more convenient for us as keepers, but these guys are a little different from other species that mainly feed on mammals & birds. Variety is the key & will help keep your garters from becoming deficient, and keep 'em healthy.

The longer you're in the hobby the more you realize there aren't really any absolutes. It's less about being right and more about doing right by your animals.