One more try to get you to understand. She hitched a ride on a sailboat. No extra carbon footprint. She did not affect the owner's subsequent crewing of that boat for it's future voyages.
Whereas I've thought deeply about your concept that flying standby on a plane means one has zero carbon footprint because the plane was going to fly nonetheless. Somehow magically only the prior booked passengers incur a carbon footprint, rather than all share. Ultimately I find it curious, but not convincing. Particularly, as I noted upthread, lifting the extra passenger does cost some 20 to 30 extra liters of fuel per 1000 kilometers.