Add the user's email address to the admin email

Basically, I am trying to get the user's email address to show in the admin notification email that gets sent when a user completes an order. I have the business edition, and I know I can create a custom field in the address table for email, which I did. However, we have a unique situation in where we pre-populated all of our users from an existing database, so these users are not being asked to fill in this field upon registration, as their info is already populated and they are already registered. I know I can access their email address from the user table in the database, as every user has to have an email address. I just need to know the PHP code to inject their email address from the user table in the DB into the admin notification email that gets sent. Not that it matters much, but I am trying to put it in the section at the bottom where it shows the user's billing address/shipping address.

Any help with this would be very much appreciated.

Thanks.