Yes, assuming he got a different power charger to work with the German power grid, it should work just fine. Plenty of people imported the original DS and the DS Lite from Japan before it went on sale in the US. You should be able to buy a "replacement" charger from Nintendo Europe or probably a number of retail stores.

Just tell your friend to be mindful of any import duties that may be assessed. It's not worth being charged with smuggling to save a little money on a game system.