Sprint IPhone error sending text to someone: Msg 2114

I trying to help out my mother in law with her iPhone 4S. She texted a friend with a 7 digit number. She received this error:

Message sent using invalid number of digits. Please resend using 10 digit number or valid short code. Msg 2114

She changed her friend's number to a 10 digit number and is still receiving the error. Her carrier is Sprint and they are clueless.

I know there is a solution on Droid by clearing the dialer data. Is this possible on the iPhone 4S? Any and all help is appreciated. Thanks!!!