You did everything you could, Janey, and that is what matters. many other wouldn't or couldn't have done anything. That is something to be proud of, that you could and that you did.
Feeling guilty about not managing to save his life is normal. If you didn't feel that then you probably wouldn't have tried in the first place. The fact that you didn't even know him doesn't matter. You were there, you were close to him and you could see,and probably feel, the upset his wife was going through.
There are so many 'what ifs' in life, none more traumatic then when a life is lost, any life, and that is the guilt that survivors live with. You will feel the loss just as if it was someone close to you. Talking about it will help and sometimes talking about it with strangers is easier. Keep doing that as much or as little as you need. The letting out of the words will help you in letting out the sadness that you are feeling.
Take care
NT xx