The user relationship module does only implements hook_user_cancel which is not invoked for "user_cancel_delete" method.

When an account is cancelled and its content is deleted, the entries for this relationship are not removed from the user_relationships table.
A view to display a particular users relationships WSOD's if a relationship exists between the a user and the deleted user, due to the entry in the user relationship table while the uid does not exist anymore.

Not sure if this is a bug, but would be great if someone could look into it.