But what are they going to sue based on? If you have a contract of fees for patent use, and it becomes fees for nothing, it's no longer a contract, by definition.

If the only remaining thing is a fee to not sue for something they objectively have no rights to, that doesn't sounds like a proper exchange to me. But of course I'm not a lawyer.

Did the other person win a case about their use, or did they actually get the patent thrown out? Could make a big difference when it comes to others.