German-USA Tax Treaty - help....

I have a client who serves as a short-term missionary in the Black Forest Academy Germany. Before leaving he rolled his 401(k) into his IRA (non-taxable event). Germany insists he pay German taxes on the value of the rollover. They do not recognize “rollovers” as non-taxable. He’s faced with over $15K in German taxes. Does anyone have advice or a contact who can help us?

Thank You.