This seems to be a somewhat common bug, apparently it happens to the joining player. Basically there's a bunch of stuff he just doesn't see.

I once saw my buddy get into a car that wasn't there, then he sped along, standing a few inches off the ground. Sometimes I unload on a convoy and my buddy doesn't get what I'm doing, sometimes I crash into a tree trying to avoid running over some allies, and he's asking me why I'm driving like a spaz, etc etc.
I hope this is the right place to report bugs