I actually see both sides of the argument and won't condemn anyone who chooses to open carry or chooses to concealed carry.
On one side of the coin, as Chris said, open carry might make someone intent on doing harm target you first. But on the other hand, most situations aren't a criminal having decided to go on a shooting spree but are people committing crimes of opportunity. Having worked in law enforcement for 11 years, I'm pretty convinced that the vast majority of armed crimes would not even happen if the criminal sees that someone else is there with a firearm. Example: an armed robber walks in a convenience store with the intent to rob the place of a few hundred bucks. He sees a man open carrying (who should have situational awareness and not get surprised by attack anyway). Nearly 100% of the time he will turn around and look for another, easier, target. If someone is carrying concealed in that situation, he may now be confronted with having to use his gun when he wouldn't have if he was open carrying. It's six of one, half dozen of another. Open carry might deter some attacks while inviting others. Concealed carry might give you an upper hand when an attack happens, but will not specifically deter a criminal since he doesn't know you have it (meaning the concealed carrier will have to face every armed attack rather than having some of them abandoned by the criminal). I believe, though nearly impossible to quantify abandoned attacks, the data bears this out.
Further, one of the major reasons we have the great fear of guns in our society is because most people never see them on average citizens. Unfamiliarity breeds fear. So there is something to be said for people exercising their constitutional right to carry openly, in view of the public, to help lessen the stigma of guns in public.
So I see value in both types of carrying. That said, I carry concealed nearly all the time when I'm out in public on my personal time. But I do carry open every once in a while. Whether you open carry, or concealed carry, the key to success is situational awareness. Both methods of carry have advantages and disadvantages, but both require situational awareness. I won't argue about either method because the arguments on both sides have merit. It is up to the individual to decide which advantages and disadvantages he prefers to deal with. And since it's life and death at stake, that is a very personal decision. I won't deride anyone's choice to open carry or concealed carry.
As for OP's friend. Definitely be aware of the fact that while open carry is legal, it is not legal to have a loaded pistol in your car unless you have a CCW license. Unlawful Handling of Firearms in a Motor Vehicle is a serious crime.
