Activation Lock on old phone

I had an old iphone 6S, it was my grandson's and I upgraded him a couple of years ago. I went through the device and erased everything and reset as if to a new phone. It had not been active on my account for at least a year and was paid for through ATT so it was "unlocked". I gave the phone to a friend. She is telling me that it is locked by Apple! Apparently he had created his own apple id, logged it with it and now it has locked it somehow. Of course he is 13, so he doesn't remember WHAT the apple id was or the 6 digit code to get back into the phone. I have the IMEI number because I had taken a picture of it. Since it has been associated with my ATT account, can they help me get the phone unlocked or does Apple have to do it? She lives in a different state than I do, so it's not like we can go up to the Apple store together to resolve the issue in person. I realize that this is a "protection against theft issue", but holy moly, it's like fort know and several billion dollars worth of gold!! It's an old iphone 6S and she needs it!! Can anybody point me to the right direction!