I'm not sure you're going to have much success finding a German doctor on an English-speaking forum. I know there are at least a few members from Europe, though I honestly can't remember a single one from Germany (two from Spain, one from Portugal, lots from the UK, someone from Finland maybe?)
If there's any way you can find someone German-speaking who could seek out a health forum based in Germany, I think you'd be much more likely to be successful.
Best wishes to your friend.
Zone diet on and off for several years....worked, but too much focus on exact meal composition
Primal since July 2010...skinniest I've ever been and the least stressed about food