Is there a way to allow a signer to edit their email address when signing the document?

Depends on what field you use and who the field is assigned to. So if you use the "Email " field then the email address gets populated based on the associated email address of the Recipient but cannot be edited by the Recipient. You could, however add just a regular Text field with an "email validation" that is assigned to that specific Recipient, then before sending the envelope, add the email address in manually. Because the text field is assigned to that Recipient it can be edited by that Recipient if needed. Perhaps add a tooltip or Note suggesting the field should be reviewed so the correct email address is being given for the invoicing.