add_action(
   'friends_user_post_undo_reaction',
    function(
        $post_id,
        $validate_emoji,
        $reaction,
        $term
    ) {
        // Your code here.
    },
    10,
    4
);

do_action( 'friends_user_post_undo_reaction', $post_id, self::validate_emoji( $reaction ), $reaction, $term )