This standard information needed if someone being look at by CRA. They write quite stern letter which can be scary but it computer generated. Sound like he have one of these. The letter has firm deadline and it say that if he fail produce proof they want, they will recalculate family tax credit and he be liable to repay any over payment. He likely just worried and trying get all paperwork they demanding to them as quick as possible.

He really does need what he asked for and the window for him to get information to CRA is normal quite short which why he wants ASAP.