I want to take food from an anorexic
I'm distressed to hear this. Do you really plan on taking someone's food just because they're anorexic?
They're not going to get fat from one plate of chocolate and I know they won't eat it.
Taking someone else's food is disrespectful. Instead, you should buy your own food and encourage the anorexic person to eat healthy.
She didn't buy that many chocolate and it's not like taking a bite can do anything bad to her.
Even if you have only taken a bite, it's still someone else's food and they may not have given you permission to take a bite and it is something that you should be apologizing to her or better, replace that food that you have taken. If the person is anorexic, then they need more nutrients to get better, and you taking their food disrupts that progress so I hope you don't do it again.