It is totally nude all the time. I'm sure if she wanted to wear it to the bathroom, that would probably be fine. You're suppose to drop everything when you arrive on the island and you don't put it on until you are waiting for the boat to take you back over to the pier. It's all good. Everybody else is naked to.
When you arrive on the Island you find an area to sit and they expect you to disrobe with in 10 to 15 minutes. If not they will ask you to leave. They are very strict about that rule. No, you cannot wear any type of clothing when you sitting on a lounge