Add-ons makes my friend's game crash

The Cave Dweller and the PARASITE OUTBREAK Add-Ons works only for me, but when I have my friend join, his game crashes once he joins, or while his game is loading in. I spent a couple of bucks buying these items to be able to play something fun with someone and he doesn't even last at least ten seconds playing without his game crashing(the whole app crashing, which is the most common) or getting kicked out of the server.