I think Sun did it. The whole episode is about how she has done quite a few bad things. She is a lier for sure. She lied about the ballerina even though she knew the maid would lose her job, she told Jin last season that she had never been with anyone else. She could tell that Sayid was lying because she knows from experience. Perhaps she was afraid of what Jin would do to her if he found out that she was cheating on him.