Short version: that message comes from the activation lookup, not from a broken install, and the reset is done on your side (your WooCommerce server), not from inside the compiled EXE.
Why "invalid email - order not found" appears
With the WooCommerce Integration Kit, the protected workbook sends the email (and/or order number) the user types in the activation dialog to your server, and the kit looks up the matching WooCommerce order to authorize the activation. Once your client changed the email, the address they now enter no longer matches any order, so the lookup fails with exactly that message. The EXE itself is fine, the order simply cannot be found for the new email.
Fixing the immediate case
Easiest: have the client enter the original purchase email (the one stored on the WooCommerce order), not their new address. The kit matches the order email, not whatever address the customer uses elsewhere now.
Or update the order: in WooCommerce, open that order and change its billing email to the client's new address. The lookup then succeeds with the new email.
Resetting the activation so the client can activate again
The kit records the activation (device token and activation count) against the order on your server. To let the client re-activate (same PC after the email fix, or a new PC), reset that activation on the order from your side: in your WooCommerce admin, on the order, clear or reset the recorded activation count / stored device token for that license. That frees the seat and the next activation attempt succeeds.
By default there is no "reset" button inside the compiled EXE. If you want an end-user-driven option, XLS Padlock has a Deactivation feature you can enable in the project: the app then offers a "Deactivate" command so the user can unregister the app from their PC. Keep in mind that a manual deactivation invalidates the current key and you issue a fresh one, so for an online WooCommerce flow the server-side reset above is usually the cleaner path.
