Why does my friend think eating 2 peices of bread makes her fat?

Of course you want to do what you can to help your friend. From what you’ve explained it sounds as if your friends heading towards an eating disorter, if she’s not already there. It’s extremely unhealthy to throw up everything you’ve eaten, so before it gets to out of control you need to get her some help. She may not want help, deny she has this problem, & say she’s fine..but you need to let someone know for her sake. You could talk to her parents, maybe a school counselor, or sibling. She will most likely be mad or upset with you for a while, but just keep in mind it’s what’s best for her in the long run. She will get over it and will probably be thankful you helped her after some time. Hope this helped even a little!

Not that it matters in terms of the detrimental effect it could have on her in terms of her physical and mental health, but I wonder how old she is?

Actually scrap that, it doesn’t matter, talk to her parents or somebody from her family that might be close to her. Doing this on your own could be difficult, especially if she shuts you out. If she has made her self sick before then it could really develope into a serious situation, which I’m sure you know.

Don’t worry about her being mad at you, for her own good you need to get her family involved.