I'm thinking of threatening the client that I have to pay after the surgery.
Threatening people is wrong, I don't think this will end well.
I know it but it feels good
You feel you have to threaten someone because of paying after a surgery. How does that work? It doesn't, so stop that threatening attitude right away.
YEAH I DON'T CARE THEY HAVE TO PAY ME.
You should contact their insurance company for the money. You can't expect people not working in recover to be able to pay you. You might just have to write off the losses on your taxes.