I don't think you have anything to worry about. In fact even if it did turn out to be MS, as she is currently symptomless and has only one lesion, she wouldn't fit the diagnostic criteria for the condition anyway so it wouldn't be diagnosed and certainly no treatment would be offered.

The lesions, the particular places they form in the brain and spinal cord, their size and their shape are totally different in MS compared to vascular lesions (such as migraine lesions) and other conditions which produce similar white lesions.

White brain lesions are also totally normal in the brain. They tend to increase in number as we age. This lesion may be absolutely nothing more than your wife's age.

I think you both should stop worrying and crack on with living.