It´s often when people have grandchildren that they spoil them. It´s a fact of life, grandparents have already brought up their offspring when they were young, and now, it´s high time for them to satisfy their grandchildren´s desires and not to scold them.
Yet, a very serious health matter is what your daughter has. Were I in your shoes, I wouldn´t avoid the visit, after all, we are talking about her grandmother and their relationship is a must. But, before going there, you should have a conversation in order to convince her of the importance of keeping her away from chocolate. You could try to explain her that not only does chocolate gain weight but also, creates a feeling of anxiety and what not.
What is more, your mother-in-law has to figure out that you are your daughter´s mother in charge of her upbringing and she doesn´t have any other choice but to accept it and care for the health of her granddaughter, if she really loves her.