Part of compassion is recognizing when you do not have enough energy to commit. If you are too tired to give your all in such an interaction it's kinder to avoid it than to struggle to listen and respond "just because you are a nurse".
Sometimes you just can't, and I think the point here is that just because we are nurses, doesn't mean we should, or are even able to, be there for everyone else at every waking moment. Sometimes it's kinder to recognize when your intervention would be potentially more harmful than helpful.
After a night shift, or a busy day, I need to turn off my inner nurse and just be, and that's not selfish, that's just a fact. I'm tired. I honestly cannot give that man what he deserves. My need to have quiet time is as essential as that man's need for somebody to listen, and in that instance, I know I can't listen. If I had to be "on" 100% of the time simply because of my profession, I would be on the fast track to a breakdown.
So I disagree that it is our duty to always give to others regardless of our circumstances. Obviously each situation is different. For me, if I can't shut down and have that time for me, I would not be able to be there for others when it matters.
That guy at the restaurant, sure, he likely needed somebody to listen, but it was not going to ruin his life if that nurse wasn't it. If I came across a horrific accident scene on my way home, you bet I would be there for the wife whose husband is receiving CPR. You pick your battles and you pick your moments, and you recognize that you can't always be present in every instance, and you do what you have to do "for you" so you can do for others when it's needed.
I would not have engaged that conversation either, and I don't think that means I lack compassion. I don't think it means I haven't done "my duty" as a nurse.