I had a guy... a very expensive guy as a matter of fact, handling ground work for me and he didn't get the oil cap on his 460 tight... dumped the whole reservoir on the customer's curb and driveway. We cleaned it the best we could but there was a stain. I knocked $100 off the price and the customer was satisfied... still sucked though.
I don't see how a stain warrants replacing concrete, it's not damaged, only stained. Cosmetic damage is different than cracking or pitting it.
I would definitely document the whole deal, get a lien if possible and do your best to clean the stain without spending more than half the job price, document that too. Then, if he wants to go to court, go and show you've done due diligence.
If the judge sides with the customer anyway, use muriatic acid... on the customer's face.