I'd replace the tool.
Especially if the plumber was a good employee. A simple act like this can go a long way to making an employee happy.
I'd also talk to all my plumbers about supplying cordless tools to them. Yes, they are expensive to invest in, and expensive to replace, etc., but if they will actually increase production, then its a no brainer. Hell, even if they only increase moral it would be worth it. Keeping happy plumbers is half the battle in this business.
We're lucky in that most areas require our field employees to be licensed. That is not true of other trades. This limits our pool of candidates and puts us in the position of making our companies good places to work for.
BTW, as I understand the law, you can't have an employee perform a task for the company without renumeration. Your tool, your employee, your cost to recover said tool, even though the dingus left it on the job. Take care when you tell an employee you're not gonna pay him to do his job, which in this case, was to recover your company property.
Disclaimer: I ain't no lawyer and I ain't never played one on TV. This ain't legal advice, its friendly advice from one business person to another about keeping yer fanny out of a possible crack.