As you checked the multiple sclerosis box, does your wife have MS? If so, certainly hot/humid conditions often worsen MS symptoms. Some even to the point where people literally have to move to a more moderate cooler climates. Unfortunately the bitter cold can also cause problems.
From a Chinese perspective muscle spasms are usually related to what we call wind, dampness and/or blood deficiency patterns. Wind can be the result of internal heat (not body temperature per se, but a deeper meaning in TCM). Basically everyone has some sort of internal imbalance - significant or minor. When the outside environment (in this case damp-heat) matches the internal environment (possibly damp-heat as well, or wind-heat) people may see an increase in their symptoms. Certainly hot/humid weather contributes to muscle fatigue and spasms in many people this time of year - even those without MS or other neurological problems.
While somewhat technical if you read over the TCM/Acupuncture Treatment page for Multiple Sclerosis you can see some of the related signs and symptoms of the patterns that lead to MS. At least some of the treatment points there would be appropriate for acupressure to help calm down the spasms.