The M3 really isn't the car to get to blend in. It's very visually loud and people notice it all the time, at least as much as a Porsche 911 (that's speaking from experience extensivly testing both and getting about equal attention).
The M3 also doesn't make a good car to drive in place of a faster more expensive toy because it gets gas mileage that's so terrible, it will kill all endangered tree sparrows within a 100km radious just through proximity alone, it makes Hummers cower in fear and some say each M3 is built with two engines, one to drive the car, and the other to just burn gas sealed within the DCT, which is why dealers aren't allowed to open DCT's on site.
In any case I'd say the 335is is the best BMW to blend in as a daily driver, while still being fun. I'd recommend he get the 335is convertible with all the options and de-badge it. People will think it's a $40,000 USD 3 series, but it's still amazingly fun, comfortable, and 90% as fast as an M3 for 80% the price with 60% better real world gas mileage.