My was a raw newbie who likely wasn't going to get much in the way of even basic training. The controls of an 870 are easy to learn, shotguns, for some reason are more politically correct (important for some, not for me and most of us I imagine), and the stopping power of a good defensive buckshot load is more than what you'll get from an AR.
AR's have more difficult controls (again, this is trying to look at it from a raw newbies point of view) and might be more intimidating to try to learn to shoot.
Having said that, I personally wouldn't have recommended either of those. I'd recommend he goes to a local range that offers rentals, talk to the guys behind the counter there, and find something he can shoot well. Oh, and take at the very least an intro to handguns course.
But between an 870 and an AR, I'd recommend 870. Adding in all options, neither of them would crack the top 20 probably.