When you said he had to pay full price for a replacement I automatically assume his first phone was subsidized by the carrier and he is still under contract! I'm also assuming the one he paid full price for is not under contract! If that is in fact the case, why would the carrier unlock it? They would then have two phones out there unlocked and easily taken to another carrier!
I'm not sure but, I don't believe I've ever heard of a cell phone company being able to lock a phone. They come from the maker already locked to whatever provider they are sold to but I've never heard of a provider being able to lock one brought in by a customer. Doesn't mean it can't be done but I've never heard of it being done!
Sprints policy is pretty clear on unlocking:
The device is Domestic SIM Unlock capable
Any associated Service Agreement, Installment Billing Agreement, or Lease Agreement has been fulfilled including payment in full of any applicable early termination fees or end-of-lease purchase options. Because leased devices are not owned by the user, the end of lease purchase option must be exercised and paid in full before the Domestic SIM unlock is completed.
The associated account is in good standing
The device has not been reported as lost or stolen, associated with fraudulent activity, or otherwise flagged as ineligible to be unlocked