Did you previously have an iPhone?. If so it's due to not turning off iMessage. If you never had a iPhone before, just would suggest having them turn off iMessage and remove your contact information from their phone. Then re add it and see if that will fix it.
Launch the Settings app, tap on Messages and slide the iMessage switch to the Off position. After completing the steps below and verifying your phone number has been removed from Apple's iMessage servers you can turn the service back on on either device, using only an email address for sending and receiving messages.
One thing I would suggest is having her back up the iPhone on iTunes and performing a factory reset on he device. This should remove anything that is causing this. She can always restore everything back on the iPhone after.