Is my friend starving herself?

I have a friend who is about 5'7 and about a month ago she started thinking she was fat. We all were at her pool and she said she didn't want to swim. I think she felt self conscious abou her weight. Then she wasn't in school for a few day and when she got back all she was eating for lunch was celery. Once she "forgot" her lunch and when I offered her some goldfish and half of my sandwich she refused. I kept asking if she was hungry and she said she would eat at home. When we were all at a party there was pizza and she said she didn't like it even though I have seen her eat it. Then two things happened in the past week that really made me worry. On Monday she was over at my house and when we went swimming I could see her ribs. Then she said she needed to work out so she used my moms treadmill for like two hours. And she kept saying that she only burned 700 calories which is a lot for me but nothing for her. She ate nothing do dinner or breakfast. The other weird thing that happened was yesterday we were at soccer camp and she said she didn't feel well. When she was walkin over to get water she fell over. But then got back up. About ten minutes late she was just running our last lap that we needed to do at the end of the day for camp and she was way ahead and she stopped and grabbed onto a bench but then fell over and did not get up for about 25 minutes. She didn't even remember what happened when she woke up. My coaches said she fainted and told her parents who didn't even seem worried. When I told them about her diet they said it wouldn't be bad is she lost a couple pounds! I can't believe this because she weighed 113 pounds when we had to get weighed for soccer camp. That was a week ago and she looks thinner now!! What should I do? Do you think this is a problem or am I over reacting? thanks for your help.