Unlocking Sprint Network
You could try lots of things. Your carrier (the one you pay) might know how to help. You could get service with Sprint, associate the phone with that service and then get it unlocked. You could return the phone to the person and ask them to unlock it. 

I would keep good notes on what you try and then reach out to your local consumer watchdog resources. Local news like to run "stick up for the local person" stories and there are government agencies that regulate carriers that you could share your efforts with.

It's a human policy issue though in the end, someone with access to a shared database needs to make an entry to unlock your device since they all cooperate and track which devices are locked and Apple doesn't really get involved in this according to any report I've seen.

Worst case, you could recycle that phone for cash or resell it if your time is worth more than the effort it would take to get it unlocked.