Of course there is always the option of non-premium trains. I don't like to call them "slow", because if you (quite reasonably) demand good travel services, you need many places to get on and off, and choices of speed and seating. For a young person's first trip to Europe, this board's usual warnings about excessive hotel changes apply less. If he has the slightest interest in a city where he has to change trains, he could book a cheap hotel in the general area of the train station for 22 hours in that city.
Once you learn the levels of train service, and the likely "change" locations, you can use either the SNCF or the DB (yes, many people here like the German site so much they use it exclusively. I always use the country I'm starting in, however.) to list legions of choices and see how long they take. Then you click on a link like "Details" and it shows you all the stops and changes.
It's only an introduction, but our host has some free travel tips. He could also go to the library and look at any of Rick's guidbooks to get an idea of train travel basics.
https://www.ricksteves.com/travel-tips
EDIT: I haven't checked this specific route, but many premium trains are sold like air tickets; That is, in exchange for non-changeable, non-refundable tickets bought at least six weeks in advance, quite reasonable prices can be available. Also, departure times less used by business travelers can cost less.