But if he's stubborn.
There is a two part plastic repair epoxy type material (it isn;t epoxy) that usually works pretty good.
Clean the bejesus out of the break and surrounding ares. Stick it back together with the two part stuff. Let it cure.
Then spread athin layer of the two part stuff around the broken area and reinforce it with woven glass cloth. The thin stuff would be best. Work the cloth into the two part stuff, then spread a thin layer of the two part stuff over the cloth and let the whole mess cure. Try to lap the cloth about an inch each side of the break where ever you can.
After all that, there is a good chance it will work. but prolly not as good as Chris's solution.
They should be more than willing to part with one of those, they are are all over the place in most yards and I bet usually just end up scrapped.
Also snag another clamp or lightly squeeze the little band on that one w/ pliers to keep the screw part from backing out like that.
I'd take a 1/8 x 3/4 CRS piece, and bend it to fit the vertical profile of the air box, attaching it with holes drilled in appropriate places. The bottom part gets bent out and a hole drilled so you can screw it down to the hole in the lower half.