add_filter(
   'friends_friend_private_feed_url',
    function(
        $feed_url,
        $friend_user
    ) {
        // Your code here.
        return $feed_url;
    },
    10,
    2
);

apply_filters( 'friends_friend_private_feed_url', $feed_url, $friend_user )

apply_filters( 'friends_friend_private_feed_url', $comments_url, $friend_user )