add_filter(
   'friends_comments_content',
    function(
        $content,
        $post_id,
        $friend_user,
        $user_feed
    ) {
        // Your code here.
        return $content;
    },
    10,
    4
);

apply_filters( 'friends_comments_content', $content, $post_id, $friend_user, $user_feed )