Ok So I Live In A Place For Kids With

Ok so i live in a place for kids with mental ilnesses and my best friend also lives there (I met her from being there) there lives 8 currently. And she's a very sweet person but she lies alot which makes me really sad. Sometimes it's small things like she would show me something that she told me she made all by herself and then I find out that she didn't actually make any of it. She also tells me alot of stories that aren't true. And then when I speak with other people about them they tell me that she made it up. But sometimes its alot bigger lies (She has an eating disorder and tells me alot about how hard it is for her to eat and that she almost never eats. And she also makes people believe that She doesnt eat candy or unhealthy things). Some time ago she told me that she was keeping a cat in her room and that I couldn't tell anyone because then it would be taken away from her and it would get treated badly. And every time we were out grocery shopping she would ask me to buy stuff back for her cat because it was hungry. But she told me not to buy cat food because that would smell in her room and then the adults taking care of us would find out she had a cat in her room. So she made me bring back nuts, cookies and all kinds of stuff cats don't eat but I didn't know that and I just payed for everything every time because I wanted to help the cat that she said she had (she also told me that I had to say all of those things I bought was for myself). This went on for about a month and a half when I then found out she never had a cat and she was eating all the things herself. and when I found out I was kinda happy because I knew she was eating and not starving herself but I also got really sad that she lied about something like that. Ever since that she still tells me to buy things for her which is mostly unhealthy stuff like salted peanuts, cookies and soda and she always says that it's very important that nobody knows it's for her and I can't tell anyone I have to say that it's all for me. I have also witnessed a couple of times that she "stole" food from the kitchen when she thought nobody was there. And I just don't understand why she wants people to think that she doesn't eat anything when she actually does. Also when she knows I have soda or candy in my room she would hang out with me and then when we ate it all she would leave and that sometimes makes me wonder if that's all she hangs out with me for. But I really love her and she's my best and pretty much only friend but it just makes me very sad when she lies to me and I don't understand why she doesn't want anyone to know she's eating and I don't know what to do when she tells me that I can't tell anyone I buy food for her because I think it's really good that she eats but I also think it's important that the adults taking care of us knows it so what should I do?