She did not get a McDonalds kettle and pour just boiled water onto herself. She asked for a cup of coffee, paid, took the coffee out to the car and got in the passenger seat. They drove for a short while. They stopped the car. She removed the lid and spilled the coffee.

A more fair comparison would be if she had poured her boiled kettle into a mug, carried that through to another room, and spilled it on herself as she sat down.

It's likely that in the latter case the water which would have been cooled by the mug and a metal spoon would have been less hot. That would have given her a few more seconds to remove the scalding clothing, which would have helped to reduce the severity of her injuries.

Part of the problem is that McDonalds was serving coffee hotter than people would have it at home because McDonalds wanted it to stay hot for the duration of the journey.