THey are usually a muffler... when used properly on say, a Subaru WRX(and most import cars) they a replacment muffler that is located near the end of the exhaust system... I don't think it would do any good for your friends car, joke or not... just stupid.
An exhaust tip, is all that it is. Has nothing to do with the muffler. Usually, the muffler is removed, to enhance the flow of the exhaust, and the tip, just amplifies the noise. You can tell he likes imports, to use that kind of vocabulary.