Add frontend “Restore” link

Question

From your comment above, I believe you’re running into issues with the _wpnonce piece of the puzzle. Looking at the code in /wp-admin/post.php, it appears that the untrash instruction is checking for a valid WordPress nonce, and not getting one.

This might do the trick:

<?php
function wpse_95348_undelete_post( $post_id ) {
    // no post?
    if( !$post_id || !is_numeric( $post_id ) ) {
        return false;
    }
    $_wpnonce = wp_create_nonce( 'untrash-post_' . $post_id );
    $url = admin_url( 'post.php?post=" . $post_id . "&action=untrash&_wpnonce=" . $_wpnonce );
    return $url; 
}
?>

It uses wp_create_nonce() to generate the nonce you need and admin_url() to get a proper wp-admin URL.