I am not a tax expert so please assess my response accordingly.....
I also have dual citizenship (US and Canada) and now am resident in Canada so I've had to research and answer the same questions. Here is what I have found:
- All US citizens must file a US income tax return every year that reports worldwide income regardless of residency
- As a non resident your friend would most likely use the 1040NR form
- There is most likely a tax treaty between Germany and the US that reduces (but likely does not eliminate) double taxation. I'd suggest getting information directly from the German taxation department
