Don't put it back in (unless you weld it in
). If that falls out on the highway it could destroy someone's tire, bang up their sheet metal, or even kill someone if it gets kicked up and goes through a windshield. The guy needs to go to a spring shop immediately and have all the springs replaced, anything else is dangerous and irresponsible.
I would have the springs on both sides replaced if it was mine. If the one on the left broke, the one on the right won't be far behind.