I hope both consult their CPAs for tax advice. The share given to the friend will be subject to gift tax.
I’m sure they can minimize the tax bill if the transaction is structured a certain way.
The unified credit for 2020 is $11.5m. That means he could gift $11.5m before incurring any gift taxes. Since he took the cash option of $16m, the half he gave to his friend will not result in any gift taxes. His risk is that the Dims get in and drastically reduce the unified credit. In that case, his estate might have to pay estate taxes on what he has left. Seems fairly unlikely, though.
I think you can claim the prize with another person, but certainly worth making sure. A stitch in time saves nine (million bucks).