add_filter(
   'notify_about_new_friend_post',
    function(
        bool $true,
        $friend_user,
        $post_id,
        $user_feed,
        $keyword_match
    ) {
        // Your code here.
        return $true;
    },
    10,
    5
);

apply_filters( 'notify_about_new_friend_post', true, $friend_user, $post_id, $user_feed, $keyword_match )