IPhone 4s (Sprint) on tmobile network?

I've read a couple of articles and forums on this subject but my case is a little different. I bought an iPhone 4S from my bestfriend so I can give it to my boyfriend for his birthday. (He has a flip phone currently) lol
Anyways, her carrier is sprint. I put my tmobile SIM card in it to see if it was unlocked and it seemed to connect just fine? This is probably a dumb question but do I still need to unlock it with sprint or anything? I don't want it to still need to be unlocked when I give it to my boyfriend.

To be certain, yes. Unless you can complete the iPhone activation from a new phone screen, it probably will not work. Why? Well Sprint and Verizon use CDMA signals. Tmobile and ATT use GSM signals. Unlocked means the phone would be capable of using either one depending on the sim and programming. The tough part? Sprint by policy never unlocks their phones for rival domestic use-ever. If it isn't already unlocked (highly unlikely) you'll have to find software to do that before it will ever work on a GSM system like Tmobile's. Maybe you got lucky but until it's completely programmed and can make a call, don't trust it. Best luck.