Localbitcoins could be a good option because in that platform you buy the BTC from the users and not from the exchange. And to have lower risk with the trade i would recommend avoiding go all-in, and change that money by batches of $1M. Once you find the right trader then go with bigger amounts.
You should be really careful with this transaction because the government can tag them as money laundering, so, play it smart.
Keep in mind that the LocalBitcoins was strictly enforced KYC/AML implementation and I don't think this is a good choice for them, for sure they will be suspected of money laundering in the first place when they want to cash out at once or the worse is when their account putting on hold and asking documents where the money comes from. If there's no source of funds, it will always be suspected as money laundering, which is very risky.
The only way to have a solution, follow as suggested above, use P2P exchange like Hodl-Hodl and purchase Bitcoin little by little so that no one will be suspected for every transaction they had. In the
Holdhold FAQ, the maximum offer in a max level account is 40 bitcoin, I think that is enough for a single transaction.