There was always that risk, whether or not he had an affair. People get divorced for all kinds of reasons and you have no idea how she felt about their marriage before she learned of the affair. You also do not know how she feels about the affair, she might have some kind of private (i.e. none of your business) arrangement with him because she sees enough value in keeping the marriage intact.

Also keep in mind that divorce is not some formulaic thing. Most divorces are settled out of court and a hypothetical settlement could have terms that allow him to keep those shares in exchange for something else (e.g. maybe he agrees to pay her the cash value or agrees to a large and permanent alimony structure). I doubt the Nvidia board is worried about this issue considering the other, more immediate problems here (like the fact that his mistress is a Chinese spy).

How would firing him make any difference. He’s already got the stock.

The worst she can do is sell them off. And at something like 1.75% she would own that is not likely to damage them seriously. Besides firing him or not, he already has the stock so it would accomplish nothing.