W210 E320ís have brake pad wear sensors. Has your friendís brake warning light come on? This system is not foolproof if the rotors are severely worn, but his mechanic indicated that the rotors are only slightly under spec so this should not be an issue. I replaced the pads and rotors on my 1996 E320 (W210) prior to the warning light coming on because I was not comfortable with the brake pedal travel. I suspect my prior owner economized by not replacing out of spec rotors. Is your friend comfortable with the feel of his brakes? Brake pad inspection is not rocket science. If you have a floor jack it is a pretty simple DIY job. Otherwise, low cost inspections are widely promoted.
For insightful owners, DIY brake jobs are one of the best ways to reduce the costs of ownership of a Mercedes. All of the parts are available from FastLane at reasonable cost, and the procedure is pretty much outlined in the DIY section of this site.