add_action(
   'friends_retrieved_new_posts',
    function(
        $user_feed,
        $new_post_ids,
        $modified_posts,
        $friend_user
    ) {
        // Your code here.
    },
    10,
    4
);

do_action( 'friends_retrieved_new_posts', $user_feed, $new_post_ids, $modified_posts, $friend_user )