Your mother should say that she received the amount that the Letter 6475 says was paid to her.
If she says she did not receive it (received $0) the IRS will flag it as an error, because they did send the payment. The processing of her tax return will be delayed. The IRS will remove the Recovery Rebate credit from her tax return.
The trace will find out what happened to the payment that was sent. She cannot recover the lost payment by claiming it as a credit on her tax return.
