Roll cages in track cars...
Okay, a fair enough idea.
What type of car is it, and how far does he want to go with a cage?
A half cage will provide decent protection in the event of a roll over, and will allow the driver to use harnesses etc on the track safely. As well, it will help to stiffen up the rear half of the chassis, and allow somewhat better performance.
A half cage also has the benefit of not being intrusive into the daily driveability of the car. If you don't need access to the back seat, then a half cage allows easy access to the front seats, and you can generally keep the A/C, stereo, seats, carpets etc without having any comfort issues (no cross braces to climb over etc). If the car is going to be primarily a street car with some track days on it during the year, I'd suggest a half cage as the most cost effective way to go.
Once you go to a full roll cage, your car tends to see mostly track duty, as a daily driver it becomes a bit of a PITA. I'd recommend staying away from full cages unless he's VERY serious about track events.
Who is your friend, and what events does he attend the track with (Trackmasters, BMW Club etc)? If it's BMW Club events, it's quite likely that I know him.
I'm not really an asshole, I just play one on the Internet