It is not possible to split up a reservation like that directly on the site. I would ask your guest to contact airbnb and make them do the changes for her. What about change the existing reservation to three nights for the first upcoming week-end and ask her to rebook the two next ones ? You do not say when this guest is due to arrive. If the arrival is very close, it might be that she is trying to honour the first week-end and cancel the two others with a 50% refund once they have been transferred to later dates.