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.
It already is an internal transfer which resulted in the error message. I think exchanges should have a column after such error messages where users can easily report it to support and have them look directly into the case without having them start a complain and describe the issue.
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.
If he has a non custodian XRP wallet he can try withdrawing directly to that too, that way he can track the progress and completion of the transaction and then fund the destination coinbase address or the nephew's non custodian wallet.