I think it's ridiculous to have expected Kobo to give you the extra $50 gift card. So there was a glitch, so what?
I'm not sure what kind of family dynamic exists that a daughter can't tell her mother there was a mistake. It also seems a bit odd that a father would call and question a notification from Kobo about an error.
And they offered you a free book to compensate! Seems to me that's more than enough.