Very Simple, get 6 trusted exchanges like;
-Binance
-Coinbase
-Huobi
-Bitfinex
- Kucoin
-Bittrex
Set up your account with Verified KYC, split the BTCs into small portions and transfer to the different exchanges.. over a period of time, Put up small orders until it is all sold out.
You don't have to sell the whole amount at once in one exchange
He definitely want's to cash out for fiat not other cryptocoins