If each apartment has its own port on a managed switch somewhere in the building, seeing their traffic levels should be pretty simple with something like MRTG.
However, this seems like more of a legal issue than a technical issue. IANAL, but by trying to police the connection the owner is essentially giving up any sort of “common carrier” status he might have had (if any at all). If I were in this position, each apartment would get a static IP to get out to the Internet. If the MPAA/RIAA came knocking, I would politely direct them to the tenant who “owns” the IP address in question.