When you state that it's believed to be heart failure
has he had an autopsy or is that speculation? It would be difficult to die in your sleep when you are experiencing congestive heart failure. He would be struggling to breath with such extreme difficulty that sleep would be completely impossible. Congestive heart failure would indicate poor heart function so he wouldn't be healthy or involved in sports, but you write that there were no signs that he was having health issues. He might of developed a cardiac arrhythmia
which was fatal. From the information in your question I don't think it was congestive heart failure so I can't answer your final question. I'm so sorry for your friend and the loss of her brother at such a young age.