I know some travel insurance policies also cover medical expenses. Might be something to look into.
All travel insurance should cover medical expenses, especially travel insurance for the US where it's vital that you have medical... as a result, travel insurance for the US is usually a bit more expensive than travel insurance for going anywhere else in the world because it covers medical too (usually you have the options: 'Europe', 'Worldwide' and 'Worldwide excluding USA and Canada'). Ideally you want insurance that covers up to £10 million in medical expenses, which policies for the US usually do.
She will have to make sure that she is covered by the insurance, as what she can take out it may depend on her age and any pre-existing conditions.
So, I would look into travel insurance that is specifically designed for people over 65 (assuming she is over 65) - like Saga or Age UK - as some policies will only cover 18-64 years.
You should also check how long they will allow a single trip policy for - I think some companies specify that the trip is no longer than 45 days.