Neighbour refuses to wish anyone HNY

I think the loss of the poor lady's husband, if I may use some words of Seamus Heaney, has been a "big buffeting" to her that has (paraphrasing) 'caught the heart and blown it open.'

Why does it matter?

Just give her time and space. Though I wouldn't be including her in the family get-togethers if she puts a damper on things, ask her at another time she would probably be more comfortable then anyway.

I noticed out on my morning walk that very few people were wishing anyone a HNY unlike previous years, just opting for the usual ‘morning.

Why should she ? And why would you care ?

It's fine. Leave her to do this her way.

Don’t focus on things like this. She’s grieving
This isn’t about you

What do you mean “Come to terms with it” I wonder. “Get over it “ perhaps?”
Hint, you don’t.
Grief has as you say no time (limits) or boundaries.
I do wonder, then why you belaboured the point?
I find it hard to believe she wants others to be unhappy too, but when you are grieving, the words can sound very hollow and don’t trip off the tongue easily.
Eventually she admitted that she won't say it because for her there will be no more happy years since she lost her DH
Poor lady, hearing the jollifications , on my own last night, I can guess how she feels