I've struggle with an eating disorder for a good portion of my life and with that came laxative and diet pill abuse. I don't know if this will be of any help but I'll give a couple suggestions.
The worst thing for a mom to do for a child with an ED is to badger them. My mom would badger me about my eating habits and it just made things worse. I'd suggest not trying to scare her either. The best thing is to constantly be there for her. Because of circumstances, my mom is hardly ever there for me and that doesn't help me at all. Be there for her and make it clear that if she wants to open up to you about what she's struggling with that you are there and will always love her no matter what. I'll also suggest that you have healthy food and little to no junk food in the house. When I am somewhere with no junk food and only healthy choices I feel better and more likely to eat without binging. Make sure she has regular checkups with the doctor. IBS or other digestive problems are just one result of laxative abuse. Your body becomes dependent upon them... Since I am not healed I cannot say what will work, especially since I'm not her. But I hope this helps and feel free to contact me. If your daughter would like someone to talk to about her ED, I'm willing to be a helpful friend. Good luck! I'll be praying for your daughter!
Its not the little things in life that define you but the big things. We all make mistakes but those of us who refuse to give up are the hope of those with no hope. Help those in need. Spread hope.