How can I prove that this girl put iron out in my juice?

It happened in October wehn I was living with her and her Uncle. She told me her cousin did it in November but a month later she admitted to my sisters friend that she actually was the one who did it. What evidence do I need to incriminate her and send it to authorities? because I don't think that my sisters friend will testify against her because she's fake. But i want this girl locked up!