Agree with what @Warspite says about resting with legs up (above head if poss), doing exercises etc. And yes, some things might not be needed. Amazon one day/Prime is wonderful here. Toilet risers, agree, too!
When I had a knee op (ligament), I was living with my parents in their house. No downstairs loo etc. If I recall, I would be lucky enough to be brought breakfast/lunch upstairs (and/or I'd got in lots of snacks/dried fruit/bread/jam to keep up there). I went downstairs once a day, normally on my bum, with my rucksack full of lip balm/wordsearches/hand san on my back, and would spend several hours down there. When I was desperate for the loo and really couldn't wait any longer, I would load my rucksack up with water bottles for the evening/next day and go back upstairs. Thus I was using the stairs but to a minimum. She can adapt this to whatever she can manage, eg aim for downstairs, make a very simple dinner (ready meal if necessary), and a thermos to take upstairs for later.
They normally check you can do two steps as part of the discharge process. But then flights of stairs repeatedly are quite different.
NB This is all for one knee only. I have no idea how you'd cope with two at once.... I mean, people do do it, but it takes a bit more planning.
My Dad had just one knee (part?) replaced and ended up being in for a week (got a blood clot) (I think that's fairly common; she needs to know what to look out for). Some moaning about pain. I saw him drive for 2hours the other day though (4.5months on).
I did rip my knee repeatedly last year and contacted the local council's social services. After a three month wait, they assessed my flat and even provided odd equipment (bath board, which you use to sit on, swing your legs round), and handle for the wall.