I'm curious what the legal obligations are for the person that left. AFAIK, there aren't any requirements that they continue supporting the systems they maintained after their employment ends.

Google has criteria for domain hostage situations, so I'd say it depends on whether an employee used company funds to register it on behalf of the org.

It seems like a simple matter of providing internal communication about the domain that includes the approval to purchase to prove ownership.

If that doesn't exist, OP needs to mend some bridges, ask very nicely for a transfer, and be prepared to offer to buy the domain off its "owner".

I’d start with offering the ex employee $1000 to resolve the issue before getting a lawyer involved. Not sure what legal standing you’d have.

That's a good way to lose money in a countersuit. The former employee had no obligation to do anything right now.