I don't follow this logic. Why is an affair with a customer grounds for announcing a resignation that caused an instant 10% stock price drop?
He didn’t come clean on his own accord - he was faced with irrefutable evidence and admitted it.
How can you trust anything he says after this?
Why should regulators or other customers believe any promises made by him?
His credibility is shot - the day to day stock prices changes are irrelevant.
Because the customer in question raises questions of the CEO's judgement, plus the possibility of knowledge that the target was violating sanctions. We don't know. That's why it's a bad situation for the stock.