looking at their website its hard to say ...
Bandwidth requirements are dependant upon the type of programming. Classical music programs require much more bandwidth than talk radio. Theoretically, all things being equal, the more channels offered, the more bandwidth required, the lower the quality. In granular terms, talk radio requires about one-fifth the bandwidth of music radio.
taken straight form http://www.sirius.com/servlet/ContentServer?pagename=Sirius/CachedPage&c=EditorialAsset&cid=1014674357105
They do not get down to brass tacks and tell you exactly how much it will use max or minimum... So tell your friend to do it for one day and see how much MB he uses for those 8 hours and then see if he will go over the 5gb cap by multiplying that by 23 days (added a few days for overtime or long months) to see how much it will use.