Perhaps a bit more is the scrimshaw held in place? Is there a bezel surrounding it or is it glued? Any other kind of mechanical fastning you can see? Are you interested in keeping the belt buckle in workable condition, or can it be destroyed in the removal process?
Just an aside: I’ve always wanted a “Save the Whales” button made out of scrimshaw.
Bill Henry: now THAT is FUNNY!!!.

Ann: try a tiny Dremel tool attachnment. Good to see you back on here.
Bill, that is krewel.

Save the Whales! Harpoon a fat..., Uh nevermind

Could a local jeweler help? At least with a few suggestions, if not removing it for you if the price is right.
I would be afraid to do that...
I think you should have the customer take it to a jewler to have it removed.