What I don't understand, is, if Bell can retroactively assign him to a data plan (without notifying him) that appears to bill by the byte, why couldn't they retroactively assign him to an "all you can eat" tethering plan, e.g. something like Verzon's $59 per month plan. I think a lawyer could have some fun with this one had it occurred in the US. In fact, this is precisely the sort of nonsense the FCC should be regulating, e.g. if the carrier is not controlling access to its facility, they must bill at the lowest possible rate during the period in which inadvertent usage by the customer has occurred.
While we're at it, the FCC should also mandate that customers be allowed to use their regular minutes, and whatever free nights and weekends they have, for data services as well as talk.