1) The customer was never notified upon exceeding her cap. 2) The customer was not given a tool, provided by - free of charge and with explicit instructions - on how to monitor their bandwidth usage.
It's completely inexcusable for the ISP to try and cast any blame on the customer here, notwithstanding the fact the user's router was also secured. If I were the customer I'd file a lawsuit against the ISP.
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.