Unfortunately, I'm all too familiar with navigating the Vienna and Prague stations with a wonky knee. I injured my knee quite badly in Vienna two years ago, and still managed to go to Prague for the second half of my vacation.
It wasn't easy. Here's what I know:
As others have said, the stations themselves are not huge. But it's never easy to find lifts. They are often in a corner somewhere. So you walk a lot trying to find them. I find this to be true in most European train and subway stations.
OBB will definitely provide wheelchair service. You can request this online and they will meet you at the ticket counter and take you to the platform, and then meet you at the platform and take you to the station exit at your destination.
While this was tremendously helpful, you still have to get yourself and your luggage onto the train. This is definitely the hardest part, and you definitely need help. As a short person, I find train stairs difficult even with two good legs!
There can be a frightful crush and rush at the door, with impatient travelers forgetting their manners. The luggage storage areas get full quickly and the train may start moving long before you're settled. This part was unpleasant and painful. I wish I could say it was a breeze, but it was pretty awful.
My reserved seat was not on the upper level, thank the gods, but this was my first OBB trip and I didn't realize that some trains have two levels. I hope it's possible to check the seat location and carriage configuration online.
The OBB wheelchair person took me to the main exit of the Prague station when I arrived. However, that's not where ride share companies pick up passengers. I learned this the hard way, after a few Uber drivers canceled because I wasn't at the correct pick-up spot. Eventually I had to limp to another entrance, which involved navigating up and then down a ramp, exiting the station, and walking a fair distance to the street.
I had the smaller MaxLite spinner and a backpack. I'm grateful I didn't have anything bigger.
So, richardtausch100, I don't think you're overexaggerating. The train will definitely be difficult for your friend. Not impossible by any means, but difficult. I hope that kind people help her and that she has a fantastic trip.