A.,
I have a friend in almost the exact same position except she is too skinny, can't keep weight on etc... b/c of so many medical problems. She finally divorced her beater husband, but her kids now have serious issues and her oldest of 3 jusy got diagnosed with a rare nuerological disorder.
My point is that I have always been very open and honest with my friend. I see her and talk to her everyday. I told her the truth, which was that it is very hard to see the people I care about the most hurting the way that she hurts and if there was anything I could do, to please tell me. Then, and here's the important part, DON'T WAIT FOR YOUR FRIEND TO ASK, just start helping. In small ways like in your friends case, bring her lunch but make her a salad or healthy eating choice and you do the same. Invite her to walk or work out and keep on her. If she says no, just invite her the next day and the next and so on. She will eventually say yes, believe me.
It is hard when you don't like the husband, I know, so invite her over to your house. Invite her and the kids to the park with you. Make it a time when her husband is at work so there are no conflicts.
Most important...Constantly give her love and reassurance and if you are 2 peas in a pod like you say, then I am sure you have already opened up about personal things to each other- so don't be afraid to tell her how you feel about her health, and show her you care.
Jenny