We are sorry to hear about your mother. Hopefully she is doing well now and recovering quickly.
Because travel medical insurance does not typically cover for pre-existing conditions, it is unlikely that a new policy would cover for a procedure that has already taken place.
You may want to negotiate with the medical provider. Here is an article that may help: Dealing With Cost and Billing
Unfortunately, medical costs in America are high. So, when buying travel medical insurance, it is best to select a policy maximum that is sufficient in case of any emergencies. This will lower your liability.
Insurance rates are regulated by law. You can't find the same insurance plan for a lower price anywhere else