How to edit stop time in Health app's symptoms tracker?

All categories are like that, not just Symptoms. You cannot change a past weight, blood pressure, step count, etc. Except of course with the workaround to create a new data point.

I don't want to presume anything, but there might be a (legal?) requirement to prevent data tempering.

I gave another look at the Symptoms category, and I noticed that there are two fields above the date called Presence and Absence (in French, I don't know what the texts are in English), which basically track the start and stop you're looking for. Just checkmark the correct field and don't fiddle with the time.

Would this work for your wife?

Unfortunately, the only way seems to be to write down the initial start date and time and create a new event with this data and new end date and time. Then, delete the initial event.