I'm afraid my wife will start to starve herself what should I do? (please read description)?

My wife has a lot of medical problems including asthma, heart murmur, dizziness, and shakiness. Just need to get that out of the way right off the bat. Anyway a few months ago she wanted to go on a diet. I don't think she's big at all but she is obsessed with losing weight. She wanted to start taking diet pills and I told her she should ask her doctor first because of all the medical problems she has. So she made a doctors appointment and the doctor put her on metroformin pills and told her to eat around 1200 calories per day. We even went to the mall and walked around a few times a week for exorcise together. That well well I believe and she even lost some weight. But then it wasn't enough for her. Now she wants to go on another diet with HGC drops where she will only eat 500 calories a day. I have begged her to at least call her doctor and talk it over with him first but she refuses. What should I do?