You cannot prevent one or more of your friends from being a proxy for someone that is not one of your friends. On the other hand, you can log everything, which

You cannot prevent one or more of your friends from being a proxy for someone that is not one of your friends. On the other hand, you can log everything, which renders propositions like "I can trust friend X not to share resource Y" falsifiable (i.e. scientific/testable). Personal computing is the thing that is antagonistic to DRM, where "computer" means "can simulate anything, including a better computer", and where "better" means a computer that does what I want, including ignoring DRM.