I bet that dude is sorry he didn't just hand it over when you pulled along side him. I sure as hell hope that your customer fires him...he's a thief. Who wants a thief on their property?
I know you said the spreader was in your truck, but just a side note to everyone. Never leave your equipment on the curb. Items left on the curb are generally considered abandoned property and can legally be taken! There are some exceptions. In some areas items left on the curb become the property of the municipality.