I checked the purchase key in question which you've added in the secure note, there is no other username associated with that purchase key.
That shows the purchase key is free and your client needs to use that purchase key to sign up for the Themeco account. The customer should go to this link and click on the create account button at the top right section of the screen:
Please ask them to make sure that they clear their browser cache before creating the account.
After the account creation, the customer can go to the licenses page and follow the steps mentioned in this article.
If the problem of the white page is happening in the WordPress dashboard of the user website installation, that might be because of the server related issues, the customer will need to enable the debug mode of the WordPress to see the actual problem error message instead of the white page.