I read your OP and nodded all the way through - having done the cross-the-planet flight way more times than I care to remember, and most recently with very small children, the thought of doing it alone, an idea which once filled me with dread, now sounds like utter bliss.
Books, movies, some shut-eye, trips to the loo sans audience, a G&T or two - and no interruptions...
However... I probably would agree to it. This is an elderly woman, not a baby or a toddler. Yes, there might be a fair amount of faffing getting to the airport, checking in, boarding and disembarking, but once on the flight I imagine she will be pretty self-sufficient. Plus, by that point, there's cabin crew to attend to her needs.
I'd probably do it - enabling someone to have such an experience (assuming she genuinely, really does want it) is pretty priceless.
I would, however, tell her to book her own Mum on the flight - what are you, her PA?!