Unless he's riding in goathead thorn territory, slime isn't going to do him any good. From my experience, those "pre-slimed" tubes use too much goop, and they go waaaay off-balance when the weather gets cold and all the goop solidifies in one area.
Tell him he's lucky-he could've had slime come out and ruin a good wheel-like it did for me! The stuff stuck everywhere except the hole it was supposed to plug!