You can definitely automate this through triggers. There is a trigger called 'Received Forward to a Friend Email'. Use that one.
Now your email is going to have to be an operational email, as by default, Marketo unsubscribes people who receive forward to a friend emails to comply with anti-spam laws. You could have something in your operational email that tries to get them to opt in.
Now the part I don't know about is referring back to the friend who originally forwarded the email. There is no token for this, so I would have to assume that this is not possible without some email scripting or a custom field.