I have the game on both my iPhone and Android tablet. I just sign into my account on whichever device I’m using. Just make sure that she has a JWA account (not a Guest account) and set to sign in with either Google Play or Facebook.
I would also suggest writing down her player ID somewhere just in case she has any problems you can contact Ludia support.
Although you can install and run the game on multiple devices, you can only sign an account into the game on one device at a time. I.e. if I’m playing on my iPhone and decide I want to play on my Android tablet, I will get a warning screen letting me know I’m already signed in on another device. If I continue I will connect (on the Android tablet) and the game will sign me out of the other device (iPhone).