Give this a go to make sure she's on the default?
1. Open your Contacts list.
2. Select the name of the contact to customize.
3. Touch the Edit button. Scroll down and select ringtone.
4. Choose the desired ringtone from the list. You will hear the ringtone.
5. Press the Save button in the upper right hand corner of the screen.
6. Now when that contact calls you will hear the new ringtone.