Easy boys.
I wouldn't of chosen that route either (In fact the kit I've got is worth around £4,000 and in my opinion will sound awesome) but without the right installation will sound sh!t.
If he's happy with it then fair play.
I will say though your system is only as good as your weakest part. Speakers in the back drag the soundstage to the rear. That's like going to a concert and turning around with your back to the stage. Music should come fro the front.
Try unplugging the rears for a week to compare the sound.
Also what size box are the subs in? Ported or sealed?