One thing that jumps out at me is that you can't be absolutely positive that it is MS that's affecting her. In such a case, I personally might say something like, "Excuse me, I see you walking in a way that indicates multiple sclerosis. I was diagnosed (recently?) and am still trying to wrap my head around it. Do you, by any chance, have MS?"

Saying the words multiple sclerosis rather than the letters has more impact, IMO. Suppose she doesn't have MS. She might think you're saying "a mess".