How would you associate the forwarded email and subsequent campaign actions to the original recipient?
You have to define what you mean by "associate."
Marketo doesn't formally support inter-lead relationships (outside of accounts, which is a different matter) but you can potentially use the Referral Form pattern to populate a field on the original lead whenever someone else reuses their link. You'd do this by posting the form twice, once on behalf of the referral (the secondary recipient) and then again, in the background, as the original recipient. It's gonna take some Forms JS API work, it's not out-of-box.