Feeling sorry for your loss, but how you dare you to use coinbase with such as big amount. Why you do not use hot wallet like Bitcoin core or Electrum?
He had to use their services to buy those coins. but yeah, he had to withdraw them to his personal wallet before sending them to the private platform.
That's a lot of money we are talking about here. @OP, am sure your problem will be resolved as far as you can prove tose coins are yours. keep all proofs safe such as transactions IDs and emails you exchanged with the private platform support (you may need it).
All you have to do now is to find a way to get in touch directly with coinbase's support team.