I never liked it when passengers opened up their own berth (IT'S NOT A BUNK!!!!!!!) because I was always afraid they would pinch their fingers in the latch. If the lower berth mattress has been made and rolled for storage in the upper, it's very easy to undo the SCA's work, meaning that it would have to be re-done. That's something the SCA won't particularly like. It's true that you CAN put down the upper by yourself, but I always preferred to do it for the passenger. In the old days, a berth key was required, and that was largely because of the fear that passengers might injure themselves. If the upper berth is in use but the lower area is set up as seating, the SCA will have to find someplace to put the lower berth mattress. That's not always easy if the car is full.
BUT..... since the original question involved a physical difficulty unique to this passenger, the whole issue takes on a different hue. Faced with such a situation, I, as the SCA, would always do whatever is necessary to set things up so that the passenger is comfortable. So I'll repeat what I've said here many times: If you have a special situation or a special request, talk it over with the SCA. Between the two of you, you should have no trouble coming up with a plan that works for you both.