Specifically, I want to change the lead source that is auto assigned to these leads, as well as the lead source (to Referral; with "forward to friend" as secondary lead source).
I would imagine this is controlled in the form (the one that pops up when the link is clicked)...but can't find it.
Please advise. Thank you!
This is something that actually can't be changed. The form doesn't actually exist in your instance because it's not a standard Marketo form.
Maybe you could have a trigger campaign to capture and tag leads when they're sent a F2F email?