What do you think killed my friend's dog?

[LovesDogs500]

My friend's dog died after falling off her bed. This is not the first time he fell, but it is the first time he failed to get back up on his own. Instead, he gasped for breath and died moments later. My friend took the dog to the emergency vet, but was too devastated to consent to an autopsy. Much of what I've read says that it's unlikely the dog's death was directly due to the fall. At most, the fall exacerbated existing health issues. But I don't want my friend to feel responsible for what happened. And so I'm here looking for answers. Here's some information about the dog. He was around 14 years old, 15 pounds, was losing his hearing and sight, didn't want to walk (which may or may not have been due to the hot weather), spent most of the day sleeping and often had to be woken up (perhaps because he couldn't hear that well), had to urinate frequently, was being treated for hypothyroidism, and was recently diagnosed with idiopathic vestibular disease, also known as old dog's disease. The idiopathic means they can't find a cause. The first signs appeared less than a month before his death. His legs would shake as if he were having a seizure. But after a few episodes, it seemed to go away on its own. He was also having difficulty standing up and would sometimes fall back down, as if the floor was too slippery. He would get warm easily and could only lay on the bare floor or a cooling pad. If you put him on carpet or a bed, he's start to pant. But he didn't exhibit any clear signs of pain either. He had a healthy appetite, would smile a lot, and showed excitement at the things dogs usually love ( car rides, food, belly rubs, etc. ). That's why his death was so shocking. This seemed like a dog who could've lived another year. Maybe his health was a lot worse than anyone realized. An online expert I consulted with said the death was most likely the product of systemic issues, which sounds like a vet's way of saying he had a bunch of things wrong internally, but they can't say what exactly. He even said an autopsy might've come back inconclusive. I want my friend to have closure though. That's why I'm looking for something more clear cut. I'm worried that my friend will always look back and wonder if this was her fault. I want to be able to say she did all that she could, that her dog's health was failing and that even if he hadn't fallen off the bed, he probably didn't have much time left anyway. I know that might make her feel much better. A sudden death of a pet is difficult to deal with no matter what. But it's even worse if you're punishing yourself with guilt.