Hatunen in :

I see at http://bahn.hafas.de that there is only one direct train
from Munich to Koblenz and it's an overnighter, so it looks like
you're going to have to change trains at Cologne.

That would be a detour and a miss of one of the nicest parts of German
rails: The romantic Rhine valley between Mainz and Koblenz!

I'd propose to change trains at Frankfurt or Mainz, using an IC or even a
slower train going directly to Koblenz. Thus Spencer might enjoy the views
of the Mouse Tower on a Rhine island at Bingen and the hilly Rhine shores
including Loreley Rock and the "Cat and Mouse" castles (between others) on
the way.

Enjoy your trip!

Paul