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.
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".