You want to sell that equity to create a loss? If it’s worth zero, I’m happy to go through the exercise if it’s marketable. This offer is only good if it’s actually worthless and I’m not stiffing you on the value.
Oh yeah! Get a lawyer. But then that's expensive. Might as well just leave it be and hold the shares.
I would simply hold it until the value goes to zero. In the event enterprise value doesn’t go to zero, I would gift any gains back to OP after taxes (because I’m trying to do them a favor, not make an investment).