Simon at E-Maps is very highly rated on the BMWLand forum. They live map it and take you out in the car, also keeping the car within tolerances, unlike a lot of other companies.
I have used DTUK on my Vauxhall Astra Diesel in the past - totally transformed it from 150hp to ~ 200 hp and major torque gains. You were able to change the boost to +- 20% and only takes ~ 5 mins to fit. They can be taken off for servicing etc if required, but they are not as individual as a custom remap. Remap's are thought to be better (but more expensive) and can be wiped during servicing. I never had any problems but ended up buying a 245bhp 330d touring as wanted it to be enough power at standard.
Do check the gearbox can handle the torque gain - the 335d was never a manual as BMW didn't have a gearbox that could handle it - whereas they had a good enough auto box.
Whatever your buddy does make sure he tells the insurance company! I was happy as bought mine 2nd hand for £150 and sold 3rd hand for £100 a couple of years later so very efficient power gain!