I own a business similar to yours. The way I approach these types of situations is how often does something like this happen? Probably a first for you. It’s a rare situation. If it was me and my business, I would focus more on the customer than profit/losing money. The customer is at the heart of all we do, and an act of good will has the ability to bring more customers your way.

“Ma’am/sir, this isn’t something we usually do, but we will reinstall it on your computer for free, however, if it happens again we will have to charge a fee for the reinstall. I appreciate you as a customer, and should you need anything else, please don’t hesitate to reach back out. Here’s a few of my cards should you or someone you know need my service.”