http://www.drdetailshop.com/magnuson_moss_act.htm
http://www.autosafety.org/article.php?scid=116&did=748
First link is self explanatory -
Another point is that an entire warranty cannot be voided based on an addition of an aftermarket part. ie. they cannot refuse service if you install an aftermarket alarm on the vehicle and then you lose oil pressure or your tires bubble.
They can void it however, if the failure is proven to be directly related to a non - factory approved addition or improper servicing.
Your friend can in fact install an aftermarket alarm as long as it's demonstrated that it was done correctly and no failures can be proven to be a result of the work. To void the warranty on such grounds they (BMW) would have to provide one N/C as an option - like the 1st link states.
rjp