I think we're all frightened of saying the wrong thing, sometimes so much so that avoiding the person seems safer.
Someone who wants to talk will be pleased if you say, "How are you feeling today?" - and then let them tell you.
But someone who really doesn't want to talk about it will shrug that off and change the subject - so perhaps we shouldn't be so nervous of asking the question, just sensitive to how it's answered.
All the same things apply to people who are ill - suffering from cancer, for instance. I know I've said and done the wrong thing for one person, having done what another person had told me was right for them.
I relate to the friends having 'compassion fatigue' thing. I haven't had a spouse die but I found the same with a lot of friends when my marriage broke down. The fact is, it takes a really long time to get anywhere near back to being on an even keel. People tend to be very sympathetic and understanding for a couple of months, but then expect you to be 'pulling yourself together' and 'moving on'. It's not that you want to keep talking about it all the time - although you do want to talk about it sometimes - but things like going out socially can be very very hard, and sometimes you just can't handle it need to leave. If your friends can't cope with that, you end up not going out at all, for fear of upsetting them - or finding new friends, which is incredibly hard in those circumstances!