Yes it is possible if your customer installs on his/her computer the Google Drive local application (see https://www.google.com/intl/en-GB/drive/download/). It then creates an additional Google Drive folder as any regular folder on MacOS or Windows. Afterwards, from the Odoo mail composer view (mail.email_compose_message_wizard_form), your customer can click the "Attach a file" button and select any document from the Google Drive local folder.