Wow, hope you're holding up well, and I'm glad to hear the man is okay. A few years ago, I had a colleague of mine collapse. I'm not trained in CPR, but I knew enough to check for his pulse, which was not there. Evidently his heart had stopped. Some patrons gave him CPR until the paramedics arrived and saved his life. They said his O2 stat was less than 5%.
Things like this are pretty traumatic to witness. Gives you a new appreciation on life.