The weight gain is not about the weight gain. The root cause is somewhere else, not in the chocolate. So first step is to find WHY the partner suddenly changed and if he/she wants to put the effort to be back where he/she was at. I would leave if my boyfriend suddenly gains 30 kg of fat (Im not talking about 5-10kg, I am talking about total body change) and wants to do nothing about it, because it will mean that our life style has to change. Now we both eat healthy, we exercise every day, we travel a lot, we do not use any medication. So his weight would reduce the activity level and life span. So yes, he will need to pull himself together and I will be with him all the way, but if he wants to do nothing then not worth it. I think the partner then does not respect you if he/she is not willing to do anything about the weight. And no, I am not talking about 6 pack, I am talking about healthy, fit, well functioning body.