if it smells like burning brakes, then check the calipers and make sure the slides aren't frozen or any of the pistons. If it DOESNT smell like bruning brakes, it's probably the grease and crap that your friend left on the brake rotor and didnt clean off now being burned.
And dont forget, brakes get REALLY hot under normal circumstances. They even warm the rims up a good amount under normal driving. You know they're overheating when they start glowing red
