Sprint iPhone 5 from Straight Talk, then back to Sprint

Community,

I recently sold an iPhone 5 on eBay. It was under contract with sprint and when the contract was up it was given as a gift to me. I purchased Straight Talk for a few months until I got a new iPhone. I then sold the iPhone 5, completely restored, updated and with no sim on Ebay. They buyer has recently contacted me saying, "Phone was made for Sprint but can not be used for Sprint anymore due to flash over from a prepay company. Unable to use as a listed Sprint phone."

Can something be done to use it with Sprint again?

Let me know ASAP please.

Thank you.