I am terribly sorry to hear this; especially when taking the time to confirm a provider was in network.
If a company/agent of that company misrepresents information and the customer reasonably relies on it to their detriment they may sue for damages (those that are "reasonably foreseeable and proximately caused) resulting from the negligent misrepresentation.
So for example if a provider is represented as in network but they are in fact out of network, the patient may sue for the difference.
Small claims is appropriate for cases less than $10,000 (i'm not sure what the difference between the in network and out of network cost is) otherwise general civil court would be required;
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The above information is for educational purposes only; if one seeks legal advice please contact the local bar association for a referral to a licensed attorney in your area that is able to review your case in a private, confidential setting and will be able to tailor advise specific to your situation.