I.G.
It is usually cheapest to purchase the insurance in the country where they are from. It will be a traveler's health insurance policy that will most likely be reimbursement based - so they will get the bill if any and then ask for reimbursement from the insurance company. I strongly recommend seeking out insurance the includes the following:
medical evacuation: they will be returned to their home country in case of a serious. long term medical problem on a medical flight if necessary. If for example one of them has an accident and requires long term rehabilitation and is unable to return to home for rehab on a regular flight, this coverage will pay for the expenses of a medical transport by air.
Repatriation of remains: yes, it is morbid, but transporting a body home for burial internationally is pretty steep expense.
They should talk to a travel agent in their country for recommendations.
My mom had an insurance policy like this when she came to visit and I believe it was a couple of hundred $$ for 3 months.
Oh, and there is no such thing as affordable private full coverage health insurance in the US... if there was we would not have the problems we are having with our health care system.
Good luck.