crisgal.......don´t panic.....She is a woman of 73 yrs of age with well established chronic disorders, so don´t look for miracles.
Google Diabetic Neuropathy...............that sounds like a prime contender for her falls and there is little that can be done for that diabetic complication.
Motor Neurone Disease is a rank outsider for the diagnosis, so please reassure her on that score.It is not necessary to see you Dr tomorrow, the next one to two weeks will be sufficient.
The description that you give of :
<<Recently, her legs have given way from under her, without warning. Twice where there has been no feeling at all, and twice where she had some feeling, but they sort of wobbled and she knew she had to sit down<<
is not accurate enough for a precise diagnosis, but taking her pre existing conditions into mind, I stand behind my comments.