Deviant Developer

On one level the cops did exactly the right thing, probably by accident. The cops almost certainly would never be able to get into the phone. So now she's wiped it they have an extra stick to beat her with "evidence tampering". Using a Faraday bag is therefore against what they might be trying to achieve (putting the chick in jail). If they'd used such a bag they'd have a phone they can't get into and no tampering charge. The only question is - are they playing 4D chess, or are they Keystone Cops. You decide.