Why Lie??

I have a friend who can’t help but lie to me about her life. It’s so frustrating and it’s driven us apart. As I’ve chosen a healthier lifestyle, she is still seeking the loose 30 pounds a month fad diets, and she is constantly trying to get me to do the same. I believe she has become so upset with her own life that she no longer goes to class and she’s gained a crazy amount of weight. I’ve asked her to do physical things with me and all she does is complain and lie about what she’s eating and how active she is. When we go shopping she refuses to try anything on and will only buy things 2x smaller than she is, to use as motivation. Then she gets upset that she has no clothes and no money. She’s bringing me down. I become depressed when I’m around her and I can’t understand why she’d lie to me. I think this friendship will have to end if she doesn’t open up soon. I’d love to help her, but I can’t be honest with her if she can’t be honest with me.