Remind him that if this does become an IDB, he can insist on compensation in money (check) and not a DL voucher. Remind him to be careful what he signs in this case. You should school him quickly about what VDB and IDB mean, although often DL asks whether someone is willing to be bumped when the flight isn't actually oversold.

In contrast to some other airlines, DL does not use the check in time in determining IDBs. The lack of DL status, or I assume even a DL FF number and the E fare class will count against him if victims must be selected for IDB.

If you post the flight number of time and destination here, someone might be able to check whether it's really oversold and how badly.

Something else to try is for the passenger to call DL and ask whether the flight is overbooked. According to DOT rules, the airline is obligated to tell you this when asked, but they are not required to say by how much the flight is overbooked. Note that oversale numbers are highly dynamic.