Adding a new contact skips the phonebook, goes to viber
There are 2 things to keep in mind:


Is your grandma actually going into her phone's contacts to add people, or is she going into the Viber app to add people?
When creating contacts, my phone asks me WHERE to store the contacts. In my case, I have 2 options: one the SIM card, or on my Google account. She may have a 3rd option (Viber). She should select one of the other 2. (I recommend Google account).


There is also a setting on Viber to let Viber sync with your phone contacts (so tyhat Viber contacts appear in your phone, and your phone contacts appear in Viber). This should be under General