Sounds like you appreciated that your boss gave you space to learn, and understood that you made an honest mistake, but you’d fire someone who made this mistake if they were working for you?
How do you square those two things internally?
It is good to punish people who give access to production databases to people who shouldn't have it. And the guy learning MySQL should not be given that access.
Taking down prod is always a symptom of a systemic failure. The person responsible for the systemic failure should see the consequences, not the person responsible for the symptom.
You don't see the contradiction of terms there? A systemic failure is by definition not the responsibility of one person. You're saying people should be able to make mistakes. But not those people.
I suspect the solution is to find a way to make sure the consequences of the decision are fed back properly into the system directed at the right person. How to do that, I have no idea.
Even C-level people always have to have their first day as C-level and of course they will make mistakes.
The important thing is learning from them of course.
Which is why I find it unacceptable that he repeated the mistake.
Easy: (1) He wasn't my boss. (2) He allowed a person not associated his team or even the tech department to conduct potentially harmful operations on the production database without supervision. (3) Those actions resulted in millions of dollars of lost revenue and make-goods. (4) He did not coach the person who brought the database down. (5) He repeated the mistake.