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Under the Double Taxation Treaty between the UK and the USA ay tax deducted by the States is allowable as a tax credit against his UK liability on the same income stream. This foreign income should be declared on self assessment. Failure to declare this income could result in a complete mares nest of unpaid liabilities and penalties.
Why, the IRS are unlikely to agree a tax free transfer.
He could try, but I do not give him much chance of success.