Some things to think about
The room is downstairs so if she wishes to go to lounge or diner (she can have meals brought to room), she will have to climb stairs to do so.
She won't have to climb stairs to board train for this room but if they choose a regular room, she will have to climb stairs to get to her room.
All inter-car connectivity is upstairs. Some people that have limited mobility but who are able to walk choose the upstairs room for this reason. More enjoyable to sit in lounge with its great views and to share a table with other travelers.
The handicap room is bargain-priced and wheelchair capable for those that need it as mentioned by the_traveler.
Meals are included for all sleeper passengers so in either case, you get meals - whether brought to room or in diner. Your Sleeping Car Attendant will bring the meals to the room if she can't or has difficulty getting to diner.
There is only one handicap room per car so it might be best to grab it quickly. Cancellation up to a week ahead of time gets you full refund and cancellation within that week but before scheduled time gets you 90% refund or full credit towards future travel. If prices go down, you can call and get difference credited to your CC. You get railfare for each person at lowest published price (minus senior/AAA/etc discount) when taking any room. Room charge is NOT per person but, like US hotels, is per room but is in addition to railfare.