Here is a calendar, although it's a bit limited and nothing is near Gottingen https://bund-deutscher-orgelbaumeister.de/orgelkonzerte/
Here is what's happening in the local churches. https://kirchenmusik-goettingen.wir-e.de/aktuelles They actually did have a "organ discovery day" but it was in October. Your friend can just call them up and ask.
Jacobi church music calendar: https://www.jacobikantorei.de/index.php/konzerte/konzerte
Next week there is a nice concert in Hannover: https://www.marktkirche-hannover.de/termine/inhalt?id=333901
Very interesting organ in Kassel https://www.musik-martinskirche.de/
Your friend can also just attend any church services: If the church has an organ, it will be played during the service!
Corona rates are spiking in Germany and restrictions are increasing again, so some scheduled events may get cancelled or moved.
Best strategy: just keep your eyes on local announcements and ask at the local churches.