You can now add multiple email addresses for a single customer, and choose to send Billing/Account related email notifications to all of those email addresses.
You can add/delete email addresses for specific customers under Subscriptions > Customers > details page for the customer > Add Contact.
Fill in the form that appears, as shown below:
You will have the option to notify this additional email address for one or two of the following groups of events:
Billing - includes events such as Invoice Receipt, Payment Succeeded, etc.
Account - includes events such as Customer Created, Subscription Canceled, etc.