It is not a fad. I have been attending medical lectures for several years. (My wife has MS). The MS reseearch had pointed out that the risk of acquiring MS is halved if you take Vitamin D supplements. They have no idea why. My children are told to take it every day. Their risk is one in fifty of getting MS because their mother has it. By taking the vitamin, their risk is halved.
Is your wife in menopause? Many docs believe MS and Lupus are caused by declining levels of progesterone.