On my devices I see two separate conversations. One goes to her phone and one goes to her Mac. I can't see which is which but she has only one phone number and one iCloud account. What is causing this and how to fix?
On her iPhone and iPad both, she will need to go to the Settings App > Messages > Send & Receive. Then make sure to select her phone number on her iPad, and her associated iCloud emails on her phone. For the least amount of confusion, the selections should match.
You can configure FaceTime the same way.