What should she do.......?

My sister recently let a friend borrow her car. She did not ask why and the person did not tell. The person that borrowed her car went and robbed a few houses. My sister was interrogated by the police and was asked to call the girl (with the police listening) to basically prove she had nothing to do with it. Now the police cant find the people and are haring my sister saying they are going to articulate a charge against her if she doesn't find this girl. My question is can they charge her with anything? Also is it legal for them to har her about it? She lives in the state of Maine