Rectification request after refund is received

First of all, you need to make sure that TDS being shown in his 26AS, belongs to him. Sometimes, deductors mention wrong PAN of deductees. If TDS belong to him, then make sure that you are showing that income (on which TDS is charged) in return.

If the return was filed online and you have intimation u/s 143(1), then you can file online rectification request.