That new Flixbus line Grindelwald to Germany in summer would be better than taking trains where you have to change three times at least - probably just as quick. Always cheap too. I'm a train nut but in this case I'd consider the bus.

Until now, Flixbus only announced that they planned a new bus line from Germany to Grindelwald for this summer, but not from where in Germany. And nothing has been confirmed yet.
May be you would have to change buses at Freiburg im Breisgau.

Trains do give you better views out windows than buses and of course are more comfy. Bus would be utilitarian for Germans wanting to get to Jungfrau Region but tourists would like trains more and www.bahn.de/en always has discounted cheap tickets if book far enough in advance - probably about as cheap as buses.

tell your mother to ask her MD, but I carried some baby aspirin to use on high elevation which may be helpful. Sue

Piatus is relatively low altitude - about same as Denver Colorado about. Should not be a factor.

Pilatus has an elevation of 6,982 feet, Denver is at 5,280 (hence the Mile High designation). Not a big difference for those accustomed to 'high altitude' but worth considering for flatlanders or those with health issues.

Chances are Pilatus will be clouded over anyway - stayed in Lucerne several times and rarely saw Pilatus! Go on first clear day! Keep schedule in flux.