If the EASA licence itself is valid, ie renewed, not suspended etc, currency should not an issue. If the 61.75 was issued 20 yrs ago it will likely be a paper one and or based on an EASA licence that the number may have changed on, and so will probably need to be reissued. Applying for the letter of verification from german caa to start the 61.75 will be the easy way to determine the answer. If he gets a letter showing the licence and rating as verified, then doing the rest of the paperwork for the 61.75 is required at a FSDO when he gets to USA ( and will need a prebooked appointment) . Also - Easa medical validity wont matter as he can use faa med for the 61.75 if necessary. The only other route to this I can see is as you suggest, to get the MEP in the USA. This will need the foreign student TSA security clearance in a different category to the type rating. The hours will be usable but there won't be much real credit as the instructor will still fly what ever they want to do to sign the candidate off for the check ride.