Let's retreat back to some productive discourse.
If Tomboy911 is able to produce any crypto address that originally deposited to DuckDice, or perhaps an old withdrawal address, doesn't that show a cryptographic connection?
Signing a message would bring some more evidence for account ownership.
If they're able to find an address from 2019, then that should at least show that they own the same wallet.
That would be a solution , but unfortunately if it's the actual case that he purchased the account from the initial owner ( which probably is ) , then he could simply ask the initial owner to sign an address on his behalf.