That can't have been easy - "tyske teusse" comes to mind. There were about 14.000 German-Norwegian children from the occupation. 18 million German men served in the Wehrmacht and the Waffen-SS, they left a DNA trail all over Western and Eastern Europe in the years of occupation, via romance, rape and prostitution.
Has your wife tried this here:
https://www.bundesarchiv.de/EN/Navigation/Use/Using-specific-types/Military-Records/military-records-en.html
(https://www.bundesarchiv.de/DE/Content/Bilder/Gebaeude/berlin-reinickendorf-wast.jpg?__blob=normal&height=433&key=XL)
These guys are - since 2019 - the legal successors of the "Deutsche Dienststelle für die Benachrichtigung der nächsten Angehörigen von Gefallenen der ehemaligen deutschen Wehrmacht" (an authority which initially only informed families of fallen Wehrmacht soldiers, but which also provided info to the foreign children of German military personnel).
Not all the files are in Berlin, some are stored in Freiburg:
- Bundesarchiv, Abteilung Militärarchiv, Wiesentalstraße 10, 79115 Freiburg, 0761/47817-0, Fax 0761/47817-900, E-Mail: militaerarchiv@bundesarchiv.de
- Bundesarchiv, Abteilung Reich, Postfach 450569, 12175 Berlin, 030/18777-0, Fax 030/187770-111: E-Mail: berlin@bundesarchiv.de (u.a. Unterlagen zur SS, Waffen-SS, SA)
- Deutsche Dienststelle (WASt), Eichborndamm 179, 13403 Berlin, 030/41904-0; http://www.dd-wast.de
There is also someone in Oslo she might turn to:
Knut-Erich Papendorf
(https://www.jus.uio.no/ikrs/english/people/aca/knutp/bilder/knutp.jpg)
https://www.jus.uio.no/ikrs/english/people/aca/knutp/
He's a retired criminologist, shares the ancestry of your wife's mother (his father was a German soldier too) and has written on the subject.
https://www.apollon.uio.no/artikler/2021/4_tyskerjentene.html
I would imagine him to have retained some contacts and useful hints from his own search for his father. Since he publishes criminology- and law-related articles in German too, he's probably fluent.
If you have your wife's grandfather's name, a location where he was stationed (and roughly when) plus the fact that he was an officer, then I believe they have enough to work on. The Bundesarchiv is not fast - an answer can take almost a year - but they are "fastidious and precise" (Freddie Mercury!) plus motivated by the fact that each query has a personal fate behind it. They still have several thousands of queries each year and try to answer them all. Good luck!