I really worry about situations like this as well. People get VERY unreasonable when road rage sets in, and while I've been lucky enough to avoid anything real crazy, I have been in enough minor encounters with angry people to understand how quickly things can escalate.
My best friend is kind of like your friend from the sound of things- He gets fired up quickly, especially when someone else is being an idiot. He isn't one to back down and is often the instigator, being very familiar with the one finger wave, as your friend was in this case. One thing I've been working on lately along with being more aware is taking control of situations when I can. If you know your friend to be a little hot headed, maybe as soon as you notice the guy tailgating say something like "man I hate these tailgating idiots, probably best to just ignore them." I realize that your buddy still makes his own decision, but maybe just an innocuous statement will help temper the situation. Maybe not, but saying "DUDE, don't flip anybody off tonight, ok?" as soon as you get in the car won't likely work well either.
"Freedom and reason make us men; take these away, what are we then?"
-Sometimes your best defense is a good reminder of your common sense-
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