I had a similar connection time (70 mins) earlier this year, also with an inbound Condor flight. Didn’t have checked bags and was quite familiar with the route to the Fernbahnhof. Unfortunately Condor was delayed, and my connection shrunk to something like 30 mins from doors open to train leaving. I did make it with 5 mins to spare, but next time I will either book a flexible ticket or book a later train and just plan on having lunch/dinner at the airport. So personally I would book the 6:42 train. I really hate sitting on a plane for 10 hrs knowing that my connection is very, very tight.

Make sure they know they can use the automatic immigration gates with their German passports. And that they understand the difference between the two train stations (Fernbahnhof and the one for the local/regional trains).