This behavior sounds weird at best. Have you ever asked her if she does this with her family or friends? The only thing I can think of as an explanation, is that she may have OCD. I have worked with individuals with OCD, and more specifically, this type of compulsion (though not this compulsion exactly). If it was just food or body issues, she would have just taken a single bite out of the whole cake. The fact that she took a single bite out of every piece makes me think this might be a kind of compulsion. She may not even know it herself. The real question here is, do you think this is the kind of behavior you can tolerate, regardless of the reason for her doing it? If it is a compulsion, she is not likely to stop easily, and she has already shown she can't stop for more than a month or two. So, ultimately, it sounds like you made the right choice for yourself.