you know, i'm usually all for righting a wrong, but in this case, i feel differently.
one, the company cannot just recieve money out of the blue. they have to have an accounting of all monies in and out. getting money in the mail could actually cause them problems. then they might have already been made right by their insurance company.
First, accounting for money coming in is trivial. All it takes is a single book entry. No matter what the source of the money, the company can receive it as long as they account for it properly.
You are correct that if they had already received the money from their insurance company, there might be a problem, but it's a trivial problem. If they simply send the money back to their insurance company, the problem is solved. But most small businesses aren't going to have the kind of insurance cover to reimburse them for employee theft, so it's unlikely.
So, worst case, the insurance company makes two entries and sends a check to the insurance company (or simply endorses the check to the insurance company and sends it). Best case, they have a couple grand that they weren't counting on - which can mean a LOT to a small business.