It's not improbable at all, especially if he lived in a rural area. You can't be denied care for an emergency condition in an ER, but if you're in the ER, it's an emergency. If it's an emergency brought on by a chronic, untreated ailment, odds are you're in pretty bad shape and at a much greater risk of death than if you'd been treated for the underlying cause earlier on. As an example, if you show up in the ER with an undiagnosed malignant tumor in its last stages, you can still be saved, but your odds of being saved are extremely decreased by that point.
Furthermore, many rural areas in the U.S. do not have ready access to the most modern treatment options available. If I go fifteen miles north, as the crow flies, over the mountains I can see out my front window, those people have horrible treatment options. They are, basically, limited to less than half a dozen family doctors and a small free clinic that is not capable even of treating a broken bone. The quickest access they have to modern medicine in an emergency is a 40 minute helicopter flight to the nearest university medical center.
Our doctors, hospitals, specialists, and medicines are, by and large, incredible in the U.S. Our access to them, however, is pretty sorely lacking for a great number of people.
I don't know that he's telling the truth, and I don't know that his brother/friend (sorry, I forgot the relationship) did everything he could have, but, based on the rural area I grew up and still visit sometimes, I could absolutely see how it happens.