I'm working on a site that will launch soon that features some social networking functionality, including friend lists. We are using the Buddylist2 module and while there are some default blocks for things like "My Buddies" that show a user their buddies, there was not a similar block that would show one user another user's buddies. We were able to create a custom view with the argument "Buddylist2: Nodes author is buddy of UID" that used the following custom argument handling code:
if (arg(0) == 'user' && is_numeric($uid = arg(1)) && arg(2) == '') {
  return array(
This worked for us because the view, which creates a block, is only displayed when you are looking at a user's profile, which is a nodeprofile node authored by the user.