The resort was fully booked, which is why Ava and her sister couldn't change their reservation to separate rooms, but the ex fiance showed up and managed to get a room.
When the ladies arrive at the resort, they go up to the counter and verbally give last name and then the front desk agent pulls up the reservation right away and goes over all the details. She doesn't ask them for a credit card or ID. In no hotel ever anywhere on the planet would you ever walk up to the front desk and get your room key without doing paperwork first.