Lawyer up immediately!!
Your former employee didn’t steal a co puter, this is a serious data breach and needs to be reported. Get in touch with the FTC and any state agencies that handle that sort of thing.
Seriously, if there was any personal identifiable information you could have legal exposure if you don’t cover your ass.
At best, I would sell them a replacement at cost. They’ve been using the one you sent for 9 months with no complaints. I’d start by offering a pretty good discount and see it’ll that makes them happy.