Very difficult situation. We had similar until recently - couple in late 80s, him disabled after a stroke many years ago, she very frail. We were getting calls 2-3 times a week to help with something or other, quite often it was he who’d fallen yet again. Luckily he’s light, and he’d never fallen far or hurt himself, so dh and I nearly always managed.
Their dcs live an hour and nearly 3 hours away, and area working, so could not always drop everything. Their food shopping was organised by dcs online, though.
We have known the couple for many years and are very fond of them, but it really was getting a bit much. TBH we were on the point of saying as much to their dcs, when he fell yet again when we were out, an ambulance came, and he was taken to hospital, where eventually it was deemed that he wasn’t safe to go home without care in place. Carers now come in 4 times a day - it was frankly well overdue, and since then we’ve had no calls for help.
OP, I do think your friend needs to make it clear to their children that she cannot continue to do this, especially in the case of falls, and they must arrange help elsewhere.