I detached my left retina Nov 2011 (MTB crash) but after a few months it was eventually determined it didn't need surgery. Is she definitely having an op? An option they gave me before going for surgery (they left it a while to see if it got worse, which it didn't) was a pirate-type patch to squish the eye and fix the retina to the back of the eye.
Where mine detached they said if I needed an op there was a 50% chance of permanent reduced (by 50%) vision when fixed, but only because it was one large tear, not, I suspect age-related with your mum, where it goes thin and patchy and IIRC there will be little vision loss, if any.
It was about 6 months until it healed to the point of not having black patches / partial loss of vision / flashes when I walked into a dark room, where the retina flaps around infront of the optic nerve and confuses it. Totally painless though (except the initial week!).
Nothing to worry about TBH, from what I saw when I was back and forth at the eye clinic place was the ops go really quickly and noone stays overnight