I was given an iphone 5 when a friend upgraded to the 6. I was with a different carried but switched to AT&T. It was no big deal. I use to the gophone plan so as not to get locked into a contract and switched from virgin mobile. If your friend is locked into a contract with another network it might not be so easy..she'll have to wait until her contract ends. But yes, this is something you can do. You pull the sim card out, she takes it to AT&T and buys a new one for $10, signs up for service and is on her way.

Last time I sold my phone to Best Buy and got a ton. You/she can always do this and apply it to a new phone or gift card.

Not if she has Verizon. Might work on T-Mobile.

If she's on the same network, yes. Otherwise she has to switch.

My company allows me to get a new iPhone every couple years and I would like to give my old one (which is still in great condition and not that old) to a struggling friend. I have AT&T and she has something else. Possible?