what should i do

You really can't tell someone what to do in this sort of situation. This is something that she has to figure out on her own. You have told her how you feel and now all as you can do is be there for her as a friend whether she stays or leaves. I know it is hard to watch but she will figure it out in her own time.

I would say something but what you say has to be thought out so that it doesn't hurt your best friend more. I would take here somewhere where you know her boyfriend will not be able to interrupt. Go some place and have some coffee or tea and have a conversation first and then gently lead up to what you want to say. She needs to know that there is better for her. You can rest assure that her boyfriend lets her know what he thinks about her weight. Discuss why she thinks that she can't do any better. To sit and not say anything for me would not be a option. I would just pick when and where and say what I had to say in a kind way. If she would bring up about her weight I would let her know that I will help her lose the weight. I would tell her I would go for walks with her or whatever she thought she could do to lose the weight. I would build up her self confidence. That is what she will need to be able to leave her boyfriend and find someone else who will appreciate her.