Every time I've rented a car in Ontario, the rental agency has included $1 million of Third Party liability insurance for free. I believe the rental agency has to provide you with at least the legal minimum of third party liability (somewhere in the area of $200,000 I think). Regardless, check at the time of rental that $1 million third party liability is included, but I doubt you'll pay extra for this in Ontario.

Edited to add: If she was renting a car in the USA, I'd definitely recommend purchasing third party liability from the rental agency....the legal minimums tend to be much lower than in Canada, and rental agencies don't automatically provide decent coverage.