If she applied for and obtained the non-German nationality without first obtaining permission from the German government to obtain the second nationality, that would be grounds for lose of German nationality. I of course have no idea if she did this, but this is presumably the most common.
There is also a new way to lose German nationality: Serving in a non-German military can also be grounds (which I would guess she did not do):
http://www.auswaertiges-amt.de/www/e...t/index_html#2
Originally Posted by Aviatrix
If there is a prize for "misleading information" then perhaps that prize should go to the German Foreign Office! The above quote is actually quite inaccurate and misleading because it makes it appear as though it is ALWAYS the case that German dual nationals can keep their dual nationality for life - which is not correct.
Under current law (in force since 2000) there are two ways of being born into German nationality - by descent and by birth. Those born into German nationality by descent (i.e., children of German parents) do not have to choose. Those born into German nationality by birth (i.e., children born in Germany to parents who are residents but not citizens) have to choose between German citizenship and their parents' citizenship before age 23.
This is what I found out so far - but I believe there must be another set of circumstances in which dual nationals lose their German nationality because a friend of mine has recently lost hers (she had a letter from the German Embassy instructing her to return her German passport as she is no longer entitled to it). She was born outside Germany to a German father and non-German mother.