Welcome to Android Central! Does the G Pad have LTE capability, or is it wi-fi only? If it's wi-fi only, then you won't be able to receive SMS messages. If the iPhone user installs Hangouts, then you could receive Hangouts instant messages using wi-fi.
Yes it does have LTE. The texts I want to receive are from my 12 year old daughters phone so I can monitor what's happening. She often deletes text conversations she doesn't want me to see. Does this App Hangouts
There should be a Messaging app. Check your App Drawer and look for an app called Messaging or SMS. Hangouts can incorporate SMS, but you'll probably just want to use the basic Messaging app.