I suppose my first thought in this case was who is bringing her food for two years?? Perhaps they could stop!

I know, we shouldn't be judgemental, and that's not helpful though, sorry.

So back to your problem. Is she physically able to fit down the stairwell?
Going down of course should be easier than going up, so long as she doesn't panic and refuse to move.

I think the idea of the Fire Brigade to help is a good one, although if they could manage the deal inside rather than taking out a window.....

I was wondering if the stairway could be made into some sort of a chute, and sufficiently strong people could lower her down it??

I have seen TV footage of somebody being lifted out of their bedroom so I guess that is an option if not possible any other way. Good Luck! Be interesting to hear what the outcome is.