Look into the community for "referrals"
If you are ready to go the code way, Sanford Whiteman published a neat and efficient way to solve it here: MktoForms2 :: send form twice w/diff data - JSFiddle
Otherwise, you can capture the referred person in additional fields and create a new lead with a call to the save2 endpoint, using a webhook.
My only worry with doing it that way is that all these people should already be in our system as it's not really a referral its just a nomination of a client that we then want to drop into a sequence. Can I can us using Sanford's method if we have to, just wanted to explore if there was a way to to say if a form fill happens on this person record, check this box on person B's record.
In general, a referral has to account for an email already being in your system. There's no way to insulate against that in daily operation anyway, you just have to detect it, i.e. if the New Person activity is from the Referral Form then it was a referral, otherwise it was someone trying to get some sweet kickback $ for someone already there.
You can fire a webhook from one record to another if all you need to do is check one checkbox. I prefer to do as much as possible from the browser because it's vastly more scalable.