When you click on ‘Forward to Friend’ link on an email template, it redirects you to the below 404 error page.
The ‘Forward to Friend’ link will work only in a sent email and not on an email template. To test this functionality, you need to create an email using the template which contains "Forward to Friend’ link and then send the email to your test lead using a run action. It will not work properly with a send sample. Open the email and click on the "Forward to Friend" link. Once clicked the email will open on a webpage and then the below "Forward to Friend" box will be visible.
To know more about "Forward to Friend", refer the below document.