She needs an intervention with the laxatives. 70 tablets in 3 days is excessive and harmful. Exercising everyday isn't a bad thing...but the image you say she has of herself isn't healthy. If she's fat, she's fat. Exercise and proper diet like the rest of us are trying to do.
If she's reluctant to listen to you, try to do some research on the negative side effects of excessive laxatives and show it to her. Sit down with her, tell her it's because you care and you want her to stay sexy but not at the cost of her health or even her life. Present her with solutions. Maybe volunteer to work out and eat with her so you can make sure she doesn't get out of hand with the exercise or the laxatives, etc. Partners help.
Hope that was worth something. Just my humble suggestions, as I am dieting myself.