To be fair, it's very easy to monitor usage through Vidéotron. Here's an example :
But it's not very advertised and most users probably don't even know it exists. Especially since it was mentioned it was a 30GB cap. So it's likely to be a typical high speed connection that is likely used by around 60-70% of the users. Those who aren't tech-savvy at all.
Since there was absolutely no effort to warn her, it's obvious the fault lies on the ISP who purposely let it run up to cash in.