Smartphone Questions? Ask A Smartphone Technician for Answers
Thank you for the question. All you should need to do to fix this is the following:
Go to Settings -> Messages -> Send & Receive
Make sure "Start New Conversations From" only has her phone number selected as well as any ID's associated with her account rather than yours. If it does not have her ID's,tap the Apple ID at the top of the same screen and select "Sign Out", then sign in with her own Apple ID.
Please let me know if you have any questions and I will be happy to assist further. Thanks again!