You could try this: Submit a support ticket
https://support.avast.com/en-ww/contact/sales_other_general#pcRefunds, cancellations, billing, payment or
license issues.
The text I have marked in bold would cover your question. Pretty much all you would have to work on would be the email address used and the Product e.g. Avast Internet Security.
Depending on your location you can Phone or Email - I think the email option may be best as it establishes a support ticket with your email. Give as much information that you can to help track down your payment and reissue the license if it has gone to an incorrect email address.
Does your wife know the email and password that would be part of her Avast Account ?
If so she should be able to find the registration/subscription there and or assign it to the new installation. She should also be able to log into that account from the installation and connect the subscription.
You should still be able to download it (as a free trial) without having to purchase it right away.
Unfortunately you might run into another issue and that is in support for windows 7 I don't know how win7 will be supported by avast, but as far as I'm aware it is still supported. But Win 7 is no longer supported or updated by Microsoft.