They go see a doctor in their own country, get the diagnosis and the treatment plan down.
the doctor then teaches them to describe their condition in more urgent terms. (for example, if someone has heart disease but stable, they are taught to describe the symptoms in more severe ways, like, I am having chest pressure like an elephant sitting on my chest with pain radiating down my left arm and jaw).
The foreigner then buy a plane ticket and come over and have the taxi drive him/her to the hospital of their choice, present with the “emergent symptoms” and is admitted and treated. the foreigner then flies back home.
I’m not saying this is what happened to your parents. nor am I saying it is their intend to walk away from debt unpaid from their treatment here. Just pointing out that thousands of people do this intentionally day in and day out and there is no legal action or collection attempts.
I also want to point this out to you. If the admission for the stroke was $5000, would you pay it? of course! And in most single payer countries that would be how much the bill be.
Instead, the $30k goes back as a loss and the insurance company use this loss to justify further increases next year on the rest of us.
just another story that illustrate how broken this system is.