Hi kzink, sure hope your buddy needs some .22LR, I run several ARs with muzzle brakes and I think you might be in trouble. If my understanding is correct, you are talking about 5.56 thru a brake compared to a .308 without a brake??
Here is a pic of my most effective brake, at 400yds & with a 40 power scope, off a rest - you can watch the 5.56 bullet hit the beer can. The bbl just don't raise at all. For the shooter or someone directly BEHIND him, the noise isn't much more than normal. The poor SOB who happens to be on either side of the brake is in for a shock. It makes a huge difference where your ears are in relation to the brake.
I gotta side with your buddy! Mine can be much louder than a .30-06 or .308 fired from a rifle without a brake, if you happen to be in the wrong spot!
A little off topic & probably more information than you want, but the second loudest firearm I own is the Ruger Blackhawk in .30 Carbine. That thing will make your ears BLEED!!