Whilst I agree stairlifts are not pretty, they can be bought as cheap as £500 fitted for a second hand one. Our cheap one has run for 4 years so far.
Ours folds against the wall and does not stop our busy house functioning. I have a husband, 4 teenagers, 2 cats and a dog running up and downs the stairs all day with no issues. I use the chair and it take me 48 seconds to travel up or down the stairs.
My husband has no trouble carrying laundry upstairs past it. The teenagers do not trip on the rail. Our house is small and yet the seat doesn't block the stairs at either end.
If I was in your situation I would spend £500 for the comfort and dignity of my loved one.
My parents moved closer to me AND installed wheelchair ramps at their new local home when I got sick because they love me and want me to feel welcome and be able to visit.
I think your husband is a very loving son who wants his mother to be comfortable when she visits.
PS. Those people who have suggested downstairs toilets I would find one extremely difficult to use, as my wheelchair won't fit in nor would me with my double crutches. As getting in and turning around sounds nearly impossible. So depending on this persons mobility aids, it might not suit her either.