Upvote:4
If your niece took out medical insurance for her trip to the USA, then her medical costs will be (at least mostly) covered. It should not be necessary for her to talk about her personal circumstances. She can simply tell the billing agencies that she has medical coverage and the bills will be met. She should give the name of the insurer - the hospital may be able to deal with them directly. Hospitals are used to delays in this case and although they may press for payment (US hospitals being for-profit businesses) they are used to this situation.
Being uncooperative will not help. Forward any correspondence to your niece. Contact the billing agency and give them her address so that they do not send things to you.
This assumes that your niece became sick while in the US, and did not visit the US in order to have the treatment. It also assumes that she didn't withhold from her insurer information about her medical conditions when they gave her insurance.