When he certifies, he can put in the amount he earned.
The reduction might be enough to keep him receiving reduced unemployment benefits — but that is not guaranteed.
And if he refused to work he cannot certify that he is looking for work! It’s a terrible trap.
My suggestion would be to ask him to talk privately with his supervisor and see if he would please NOT recall him to work — and instead give those hours to someone who really needs the money (and the job benefits). If the company will still list him as unemployed, he can continue receiving benefits. The company may think it is doing him a favor in giving him some work — when, in reality, it deprives him of not only traditional unemployment but the $600 weekly Federal amount.
Of course, he risks NEVER being called back!!