There are no factory mechanism to forward a mail to a friend, and change all the tracking codes.
It means, if recipient A forward a mail to someone (person B), this person by opening/clicking on the mail would register information in the Adobe Campaign DB by using the recipient A account; the tracking information is not attributed to the forwarded person B, so usually it is an issue in CRM matters.
But you can manage it differently with Adobe Campaign: add a link in your email to the webApp Sharing Form (internal name "viral"), so when the recipient A wants to provide the shared information to person B, he clicks on the link, it opens the webApp and types the emails of his contact. The information of person B is stored in the nms:visitor table, so not affecting tracking information of person A. Moreover, it complies with law, because optin is managed accordingly (if the person B has to be transformed into contact nms:recipient).
Of course, you can adapt the viral webApp (I recommend to duplicate and change the duplicated version) and so, adding the coupon/code to provide to the contacts. Or define more or less contacts (for instance 5 contacts with first name last name email, etc).