This will really require an update of HikaShop in order to fix the issue.
We'll be happy to look for your client's account. Please ask him (or do it for him) to contact us via our contact form and provide as much information as possible on his order of HikaShop for us to be able to find it and verify it (order number, email address, name, address, paypal transaction number, etc) and we'll be able to give him back his access to his account.
Here is the contact form:
www.hikashop.com/support/contact-us.html