Hey there, I wouldn't say your worries are unfounded, first off. It definitely seems like she has some worrying eating habits. Sometimes I will eat like that, if I have eaten at home in the morning, but it's also common for people with eating disorders to make excuses such as this, even though they may not be true.
I think one way you could go at this would be to talk to her parents. They can monitor what she eats at home much better than you can, and they may be able to tell you if something really is going on. I doubt they would mind if you told them your concerns, because generally parents want to help their kids.
In terms of your friend, I think the best thing to do is to just support her as much as you can. Sometimes it works to flat out ask someone about eating disorders, but in this case, it doesn't seem like she's especially willing to talk. She might just get defensive. But, if you let her know she can talk to you, and you'll support her, then she may be willing to open up on her own terms.
I think the main thing here is to just keep an eye on her, and tell an adult if you're really worried. =)