I haven't tried it yet but I assume it's simply level 50 to you and level 30 to him. Whatever portion of health you take off with a swing on your screen, that same portion of health is taken off the zombie on his screen and vice versa. It will look as though your teammate can hold his own at level 30 against a level 50 zombie from your point of view, but from his he's just battling a zombie of his level.
I noticed that when a low level player joined my game, he was consistently doing more damage with his kicks against a level 50+ zombie (on my side) than I was. I'm assuming it's how StonedRock said, considering that kicks are far more effective against lower level zombies than they are later on.
Hardly stupid as it allows people who may be completely different levels to play with each other and not have a situation where one player is useless or the other player just dominates. It's kind of a nice option to have.
For example: I have a character that I only used when I played with my friend. He went on vacation for a couple weeks while I got the urge to do some side quests with that character. Now I can level up and not worry about being completely overpowered if I join his game or him being weak if he joins mine.