Firstly it will partly depend on the speed of progression. There is no rhyme or reason to that one - some are rapid on through to those that are very slow. Some speed up and slow down at times to keep it extra tricky.
However, I would not say that any PALS is walking perfectly one day and then not walking the next.
However, some PALS are having issues with walking (or other tasks if the limb is the arm) for a period of time and then suddenly one day cannot weight bear. It's kind of like having a tipping point. Up to a certain point there are enough functioning muscles to keep the limb working, then after that point there are not. If it's walking, the person may still have some movement etc, just have past the point of being able to walk. Maybe they can weight bear with some assistance still.
I hope that answers you. The one huge issue with this disease is that no two PALS are exactly alike, so predictions are just impossible.
Now I would like to ask - what is your wife's concern? Maybe we can address that?
I'm presuming it's about walking, but maybe it isn't. If you can tell us what limb/s, what kind of functionality the concern is over and where you are at now, we may be able to give some good contingency ideas. That way your wife may feel that she is prepared for things that may be a concern?