I would build a second survey separately, and then ask the respondent to use 3 mailto links to his/her friends. Limesurvey offers pretty cool mailto management in the link button (html editor), allowing you to set the receipient adress, subject and mail body. You will put an URL in the body of email and can try to also include the referrer token if needed.
The bad news is that mailto will only work only if the PC knows to pass the link to a mail client or webservice.
Let me know how it works.
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