Breaking own company rules, for start.

Then showing how unreliable you really are to be in position of such power, since you can destroy value of the company.

Not to mention that once his wife is done with him she will own considerable amount of NVDA stock, so much that she might even affect the price of it in short term.

They are not firing him because he fucked someone (although, in 2nd case they should).

They are firing him before he brings down company with him. Considering NVDA dropped 8.6% after hours, they might be late.

He became a huge liability. They are getting rid od said liability.