renting might not be such a viable option then, as it's around 500 pounds a year, and if it's ongoing forever then it wouldnt make economic sense - they may as well buy one. Again i urge you to urge them when they are looking at houses to consider places with straight stairs - it is so much cheaper as you can buy them 2nd hand - if they have curved stairs the rail/track is made bespoke and can be very expensive.
As i said before, this is the kind of thing DLA is meant to help pay for so it's unlikely she'll get extra help -they will say she could put her dla towards it, unless as stated befor eshe is on means tested benefits. Has she ever worked? Been in a union? beein in the armed services? If so perhaps they might be able to help her with a grant or out of a hardship fund - however, if she is buying her own house that might be difficult