Debts fall into several categories, usually ranked by likelihood of being able to collect on them. Some debt collections companies will "buy" groups of them (well, the information of the circumstances and the right to attempt collection on them from the debtor) for various amounts with the hopes that if they cash-in on even a couple of them, they've paid for themselves. This is probably one of those "shot in the dark" letters by a company that just acquired the rights to pursue his debt. Whatever you do, don't engage in communications with them -- it'll only encourage them and make them think you know more than you're letting on. Lay low and pretend you don't speak English (just kidding). If it's in his name, then you're golden. "not at this address" and send it back. They're just fishing for leads hoping they'll get a bite, they have no clue if he's still at that address or not...