Buying a T-Mobile iPhone 5 off of a guy

So I want to buy a T-Mobile iphone 5 or my little brother for his birthday and a friend of one of my coworkers is selling one for $400. Which is a great price but here is the thing that I have to ask you guys.

The guy said he bought the phone on release day and paid it off in cash. He says that after that he unlocked it through T-Mobile because of course he paid it off but after two months he lost his phone and reported it as lost to T-Mobile which he got replaced. Then he says after a week after he got it replaced he found his old phone which is the one he is trying to sell me.

So my question since he got it unlocked does T-Mobile still block the imei of the "lost phone" like they use to before the uncarrier switch or because he technically owned the old phone they can't do that ? Just need to know before I pull the trigger on this iPhone and end up having to pay extra to unlock it.