Yo the food he ate cost maybe max 15$.Find a store would be my priority, if i couldn't find one i would contact the customer to inform him,/her about the situation. And that I will pay for a new meal.
Just eating someone's meal is a no go. You cannot think i need this or i will pass out, the customer might be diabetic too.
Moral of the story, every responsible and life loving diabetic that i know carry around sugary tabs. How can an adult forget his medicine. What is next an asthmatic person forgetting their inhaler?