Simple Fix for a WordPress Techie

First of all: I know there are thousands of WordPress people out there but I need someone who knows the setup process very well.
This job will be simple for a WordPress techie, and baffling for a WordPress user.

I have a main site at http://www.zandax.com
I want to run a parallel site at http://www.zandax.training as a development platform to implement a full redesign.
Then switch the main site to point to the parallel site folder.
This is a temporary situation so I don't want any big changes to how things are set up.

The site uses hand-coded PHP scripts to access the WordPress database.
I have copied the database and it looks fine from the outside, but I get an "Error establishing a database connection" message (as you'll see if you click the link above).
I've tracked it down to this piece of code in load.php:

function wp_set_wpdb_vars() {
global $wpdb, $table_prefix;
if ( !empty( $wpdb->error ) )
// Gets to here
dead_db();
// Doesn't get to here

I can code PHP but have no experience of the inner workings of WordPress so that's as far as I can go with this.
I guess there are things that need to be tweaked in the settings/scripts, but I don't want to start experimenting.
I just need someone who can get to grips with the problem and show me (via a Zoom screen share) how to fix it.

I had an IIS problem that was fixed this way last night and worked very well - it took maybe 20 minutes.
I expect this will be even quicker.