Although we can't prevent anyone from forwarding the email they receive, doing-so will not allow for a new RSVP to your event. Since the email they forward will have the same unique invitation link that was sent to them, if multiple people use it, the replies will overwrite each other.
If you need your guests to be able to RSVP for their friends, the best thing to do is to set your total invited number as something greater than 1 so that guests can RSVP as more than 1 attending. You can also add an additional RSVP question on the Enter Details page (under the additional options menu) to ask guests to reply with their guest's name or email address.
Also see:
Can I change the total invited per invitation for each guest?