My suggestion is that you consolidate the two databases into one.
On the 'other' database, do an export, then in the WordPress database import this file, now you will have one database with both the wordpress tables AND also the other tables, this will be fine just so long as you don't have a table name in common (which is unlikely).
Now you can access all your data using the $wpdb database connection. Do this by creating a custom page template which borrows your client's code, it can do all the database entry / edit / add etc operations just like how the client stand alone pages did, all under the WordPress umbrella.
Once you get some perspective on this organisation, checkout how you can use shortcodes to help, details here:
http://codex.wordpress.org/Shortcode_API
