Can I use a T-Mobile iPhone 5?

I plan to buy a t mobile iPhone 5 from my girlfriend. Her step-dad still owes $500 on it. He plans to cancel his service with t mobile though. If he cancels the service and I buy the phone from him will I be able to use it once I take it up there to transfer my number? Is there any possible way it would not work?

It should work as long as your using t mobile.

It should work. If you are on T-Mobile yourself you could just swap the SIM after he cancels the plan & it would work fine.
If you are on another carrier you would have to see about getting it unlocked.