I recommend supplementing with either magnesium malate or magnesium citrate. Her symptoms indicate a magnesium deficiency to begin with, so part of the reason for the recommendation is to increase her magnesium levels. But not all magnesiums work equally. Magnesium oxide for example is crap. It is poorly absorbed, neutralizes the stomach acid and works as a laxative by burning the intestinal wall. Magnesium chloride ("magnesium oil") is safer and better absorbed but is less effective in elevating ATP, which is essential for fibromyalgia, than magnesium malate or citrate. Magnesium malate though is the more effective of the two though and provides other benefits such as the malic acid dissolves uric acid.
You were asking another poster about the difference between D2 and D3 in an earlier post and I would like to answer that really quick. D2 is inactive D. This is what is often added to milk and cheap vitamin supplements, but does virtually nothing for the body. It can be converted in to inactive D3 though by the liver if the liver is functioning properly. Inactive D3 is also the form of D3 commonly used in higher end supplements with vitamin D. This is because active D3 is outrageously expensive. Last I checked the raw material was over $600.00 per kilo. Inactive D3 though can be converted in to active D3 by the kidneys if the kidneys are functioning properly.
Vitamin D is important for a lot of tings including calcium absorption and immune regulation. It is essential though not to overdo it as too much vitamin D can cause problems such as elevated serum calcium that can lead to muscle cramping/spasms, high blood pressure, constipation, brain fog, weakness, etc.