t's always a difficult one. If he spends the money and they ask for it back he'll almost certainly have to repay it at fairly short notice. Even if he says nothing he'll need to keep the money to one side just in case they ask for it, so he won't get any benefit from it except some interest. And he may spend a fair amount of time worrying that they will come back and ask for it. And it could be argued that by failing to point it out he has been dishonest.
I think it's likely they;ll notice.
On the other hand the payment may be correct- so if you query it and they say "it is right- keep it" then he can enjoy it.