Maybe she’s done this nightly, for the past decade, whereas your attempts to rescue her could waken her - which, with somnambulants, can cause disorientation and violent reaction - could be the cause of her fall.

I am not an expert but walking on the edge like her would require pretty extensive prior training to achieve while awake, let alone while sleep-walking (sleep-walkers are often described to be clumsy)

My brother suffered a bit of sleep walking when he was kid, but the worst thing he did is that he would go knocking at our neighbors doors in the middle of the night.

In this case, action should be taken after she can safely be secured.

I doubt it’s something someone can do even a couple of times without ever falling, so I don’t think just waiting for her to come to safety on her own would be wise. Honestly, the one to blame would be family members if they allowed for it to happen more than once. I know I’d literally go ultra protective if it would’ve happened even once to a family member.

So solutions of the time are likely going to involve morphine or some other heavily sedating drug, or maybe some form of at least nightly incarceration, if she’s lucky

Perhaps. The easiest and most immediate solution would be to simply chain her to her bed.

In reality, I feel like in such situations, most people (including me) would suffer from anxiety paralysis. I wouldn’t literally be able to even move to try putting a cushion on the ground let alone grab her, and it’s actually something extremely interesting and yet utterly horrifying to think about.