Hmmmm. Since you are using DVC points I think I will move this to the DVC board. The mods there may decide it belongs with our Dining plan posts but I will let them decide. Hopefully some of our wonderful DVC gurus can help you out.
From everything I've been told, the answer is that no, you can't purchase at check-in. It has to be purchased at least 3 days prior to check-in. I believe your friend will need to call Member Services and add the dining plan to the reservation and you will need to arrange with her/him how to pay for it.