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.
That’s for estate inheritance. Gift taxes kick in at $15,000 for 2020. The giver and his spouse could both gift $15,000, for a total of $30,000 to the recipient before the tax kicks in.
I would think the previous promise could be taken as a contract that would entitle his friend to receive half the prize directly?