To be honest I do not like that someone holding so much BTC that they can create panic in the BTC market so easily. This is not good for crypto in the long term. I am just thinking about how much the market will be affected by a "profit booking" tweet from Michele saylor. Some individuals will hold so much power we could see misuse of it very often in the future.
You shouldn't have let him buy that much then. You should have bought it yourself until it's sold out so no one else can buy.
But hey, it's a free market. Anyone can buy bitcoins.
However, it is very unlikely that they will do any "profit booking" anytime soon. All their coins are locked up for at least 5 years.