There are several discussions on that topic, so try a search for the detail. It may look overwhelming, but those will help you through any questions you may end up having.
In a quick summary (for Android at least): First, log off the app on the phone. Set her up with a separate account under “Manage Users”. I also recommend adding her to “My Contacts”. Once she gets the email from SmartThings about being invited by you, log on to the app with that account. You’ll get basic instructions in the email, but it’s straight forward. After doing that you should be able to add her phone as a presence device.
I have never seen more convoluted and less intuitive process to setup anything called ‘smart’. Every step of the way has been pure pain. Starting with two applications and the new one being half baked. Absolute pain.