find out how the lad was payed, was it in to the bank, or cash, as if it was cash the boss might turn round and say he never worked for him, or that we worked for 1 day, lost the keys and then quit
if it was payed in to the bank then you have evidence that he worked there, and that is what the inland revenue will want as proof, weather or no there was wages slips, if they believe he worked there then he did
dont bother with acas, tell him to take the bloke straight to small claims court where he can get him ccj for the debt and then if he doesnt pay with in 28 days sell the debt to a debt company who will add there fees on it and then will send bailiffs
this is what i would do