If someone is tailgating, it is more than likely a thoughtless act by someone who is being careless. It is annoying but still typically within the control of the person being tailgated to remove themselves from the situation and make themselves safer.
Actually, I disagree. Especially in heavier traffic, someone being tailgated often cannot easily extract themselves from the situation.
Note that I am definitely not advocating throwing solid objects or anything like that. But the risk involved in a beverage is relatively low--likely lower risk than the tailgating itself.
Regarding your friends' sister--What I'm about to say is going to sound callous, so I apologize for that, but I need to say it in the way I will in order to make my points. For the point of argument, let's say that she was tailgating, and someone did this to her, causing that accident.
First, most tailgaters are habitual tailgaters--meaning that if she was tailgating here, she's done it before. Therefore, if the cops were in sufficient quantity to be able to do their job, she would have been ticketed, possibly arrested for her actions, causing her to learn her lesson, and thus preventing this incident from occurring. If she didn't learn from that, she would have been ticketed/arrested again, to the point of losing her license, thus also preventing this. Part of the cause of this accident was that no one ever previously taught her how to drive.
Secondly, and more importantly, there is a lesson here--not to her, but to others. And that lesson is, "tailgaters pay for it--sometimes with their lives". Even with the tragedy involved, the guy who brake checked her may have prevented even greater eventual loss by teaching everyone that knew her or heard about the incident that tailgating can have serious consequences. It's the same argument that people who support concealed gun laws make--people are less likely to treat you and others inappropriately if they know that there is a chance of severe, immediate consequences for those actions. One person with a concealed weapon at Virginia Tech shooting a crazed gunman could have saved dozens--and one person taking action against a tailgater may have saved dozens as well.
FYI, I'm on the other side of this discussion--I've had two friends killed by tailgaters, in separate incidents. So I have no sympathy for them when others take action against them for their actions--even if that results in the tailgater's death. Better them than us.