It's a combination really.
When an email is sent via Marketo any links within that email are branded for tracking and this branding is unique to the recipient.
This allows you to associate anonymous activity back to known leads in the database.
When the person clicks the link in the email to the LP with the form then it will be prefilled. If the person forwards on that email to someone else then the email is being sent from their email client and all the links will still be those designed for the original recipient. Therefore when the 2nd recipient gets it if they click the link that brings them to the form they will see person 1's details pre-filled.
Best option is to try to implement a forward to a friend CTA in the content of your email :
This is exactly what I was told however I have a caveat for you.
IF the user who receives a forwarded email then fills in the form with his details AND a new email address then a new lead is created.
I was on the understanding that this would overwrite the original lead details but was told emphatically by Marketo Support this was not the case and as Marketo matches on email address, it would create a new lead.
Yes, a form will update the fields on the lead with the filled-out email address, not the previously associated lead. It will also reassociate the current cookie with the now-current lead.