Hello Kli1,
Sorry to hear you've incurred unexpected long-distance charges. However, it isn't a Fido issue. Unfortunately, there appears to be a bug with WhatsApp. While it's not mentioned on their website, it seems as though the app will automatically switch to using mobile airtime minutes (within the app) if it deems the internet connection is insufficient for voice calls. It has happened to many people across Countries and even Continents (see here). One suggestion to prevent the switch would be to enable Airplane or Flight mode and manually enabling Wifi prior to making the call(s). Doing so should prevent the app from switching to mobile minutes if the Wifi internet connection is not sufficient. However, it also means that your mother wouldn't be able to make WhatsApp calls using a mobile data connection.
As mentioned in my other post, mobile providers only know usage. They cannot divert calls from within apps to cellular calls. They have no control over how apps work.
You should note these forums are community-driven and not intended as a venue for customer services. If you would like to further discuss those charges, you would need to re-contact customer service. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
Hope this helps 😀
Cheers
