Is it possible to create the functionality where, on creating a node, the user can tag friends to associate their friend with the node. Such as in Facebook's tag a friend in a post.
This would require a friends lookup field, which would imply that the users have already approved each other as friends (maybe using User Relationships).
So, the key thing is to provide a lookup field which searches the user's friend list, and also exposes itself to Views.
Ideally, the field would allow you to enter an email address if their friend isn't already on the site (probably asking too much, and could possibly make do with using the Invite module here).