Changing flight booked by someone else, using points
Let's say I book a flight for my sister, using my own Rapid Rewards points. I send her the confirmation number and such. Some time later, she decides she wants to change her flight to a different one, that costs an additional 600 points. She logs in to her account, goes to Travel Tools->Change Itinerary, and enters the confirmation number and her name. What happens next? I honestly don't know, but I imagine it's one of these options:
A) She's told she can't modify this reservation, because it was booked with someone else's points.
B) She can modify the reservation, and it automatically takes the extra 600 points out of my account.
C) She can modify the reservation, but only after I somehow "approve" the extra points taken from my account.
D) She can modify the reservation, and it takes the extra 600 points from her own account
E) She can modify the reservation, but only by taking the new full amount out of her account, and the original points cost being refunded to my account.
Which of those is what actually happens if someone tries to do this? Is there an option F) I'm not thinking of?