Just turn off iMessage, have her go into her iTunes account and disable her number to the phone, she will need to plug the phone in to be able to do this and if all else fails have att put her in contact with Apple to have them remove the number from their servers.
This whole process will take a few days as it propagates across att servers, I know this since I just had to deal with the same issue this week as I just moved from a 5s to an s5.
In addition have your friends and family remove any current iMessage threads they may have running on their devices. Though you can not have random people do this it just will take time for their phones to update.
One last issue I found was that many of my friends had me in their contacts as iPhone, instead of mobile, I assume this tells the device to use iMessage in the first place.
Make sure they edit your contact and set you as a mobile device and not an iPhone.