He has a glider license (SPL - Sail Plane License) issued in Europe under EASA's Part-FCL (Flight Crew Licensing).
He can get a "based on" FAA Private Pilot Glider Certificate dependent on his German one (including medical requirement). This is a paperwork exercise and requires a flight review with a CFI before he flies.
He has an existing FAA Private Pilot Certificate (Guessing ASEL, what the Europeans would call SEP - Single Engined Piston).
With his exising SPL and FAA pilot certificate, I think all reqirements are met for an add-on glider rating with the exception of three recommendation flights and a checkride. No written required for glider add-on.
This might not be enough "Soaring in America" practical knowledge to pass the checkride, but the minimum requirements are met. Given a dedicated instructor and a weekend it could likely be done. FSDO/etc. not required. Lots of places it could be done.
That's my recommendation as a CFI-G. I PM'd kontrollturm (Control Tower in German) with the above info. No response.
Terry