high end home surround sound systems have speakers in the back. it is true that most of the sound comes from the front/middle speakers though. I think that a car is such a small space that once your in the hundreds of watts, sound is bounching around so much that it doesnt matter THAT much. If your serious about getting great sound, then you have to go with top end speakers, and put them in kick panels in front of you. tweeters on the a-pillar and a properly tuned bass enclosure with sound dampening behind all panels, under the headliner, and under the carpet if you want to go all out i suppose. but it sounds like he just wants something that sounds good to the average person. Good bass out of his subs, and decent mids and highs for good clarity.
Now by your standards, this is a bad setup, but for the average person, getting into sound dynamics, dampening, and screwing with all sorts of different crossovers and amplifiers with dual batteries is not what they consider fun. they just want get in their car, turn the stereo on(or up if you never turn it off) and listen to music.
6x9's facing frontward towards the windows sounds ok. might give a slight reverb effect coming off of the windshield. since you will hear the door speakers first and then the 6x9's coming off of the glass milliseconds later. sounds like it will fill the car with good midrange if he can tune it right.
on a side note, you might want to move the 6x9 boxes around/different angles n such before you permanently bolt it down. kind of test out what sounds best.
