Or he can also ask his nephew to request an XRP address from his Coinbase account, and he should try sending it through there since it's the same exchange. Their system should be able to detect it as an internal transfer, saving him the fee he is trying to avoid.
I thought that's what he meant by saying he tried to send the funds from his Coinbase account to his newphe's account. I suggested using a non-custodial wallet just to confirm that the problem has nothing to do with the account or address he's trying to send to, but rather Coinbase's systems.