LOL...That's nasty.

I'd just call him up and say something like, "You know what you did was fucked up, but we still cool...I'll be packing the bowls for now on" and just let it ride. No one likes to be a victim of a practical joke...but it was by a friend, theres was no harm so no foul, you smacked him, now toke up together....but YOU pack the bowl.

EDIT: Just one question though...how could you have not smelled that something was burning besides herb?