return and have a sit down where you say "Look, this just isn't working . . . for either of us" and then give them a rough timeline of when you need to separate. Then encourage them to find other employment during that time. Better yet, work your contacts and connections to help find them other employment. (This may be supremely hard if they are being let go for truly terrible performance).

I've seen employers take this tack in the past, and it can make otherwise impossible situations a net positive for everyone involved. Your employee can have some continued coverage and a paycheck for a bit while searching for employment, and you can do well by them while at the same time mitigating some of the potential liability and almost certain blowback you'll receive for terminating someone so close to a major medical event.

Get counsel on this--from HR folks and maybe (probably?) even an employment attorney. And good luck.