It didn't "lock" his Apple ID.But why does a missed return allow Apple to lock your Apple ID. Apple ID isn't your wallet or payment method.
Companies deal with non collected funds in many other ways.
It prevented him purchasing anything else associated with that Apple ID. (I.e anything purchased specifically through Apple).
He'd been credited with money, but had reneged on his part of the agreement.
That makes additional payments via that Apple ID look suspect, so they disabled the ability for that AppleID to purchase anything on iTunes, App Store etc.