iMessage automatic deactivation

Hi, I purchased a used Iphone 3gs that I gave my mom. I plugged in her SIM card, but then she told me to take the purchased phone with me and set it up and then bring it back a week later with all the apps and settings.

She put the SIM back in her older Iphone 3G. What I didn't realize that iMessage registered itself on the new device. So day after day as I tried to message her, it would switch to iMessage format (blue), despite her not having it. After about 5 days, it finally stopped doing it, and it resumed back to regular green SMS format.

Today, when I put her sim card into the newer phone, iMessage would activate, but it doesn't send messages, nor receive and if I try to send her a message now, it no longer turns blue.

I tried googling this issue, but since the device is registered to someone else we cannot unregister the device.

Any suggestions how to get iMessage back?