Social query

Invite her as you normally would .
Plus think of an excuse to meet up in the New Year ie :
Can I ask a favour of you ?
Can you come to the January sales with me ?
I need some new clothes and I value your opinion .
We could have lunch too .

Speaking from experience I would only have wanted to see the friends I knew well at that time. She may be glad to see you as I was happy to see my oldest friends. Asking someone she isn't close to may be awkward as tears can come unbidden when newly bereaved.

Nicely put.

It's so nice that you're thinking of her. I would put it to her - she may be grateful to be just with a couple of friends that can talk about her husband with her, or she may not feel up to being sociable. On the other hand she may prefer having something to take her mind off her loss for a short time.

I would just say something like "Can you make it to ours on (whatever date)? We are looking forward to having a nice chat. We were thinking of inviting (so and so) at the same time, but didn't know if you would prefer it to be just the three of us"

Ofcourse you should invite her. You could invite another neighbour as well, just to keep the conversation varied, and cheer her up a bit.

I would still invite her . It’s going to be hard for her , the first Christmas and at least she knows that you’re thinking of her. Was it usually just the four of you or did you invite others ? . If just the four of you , you could mention that the other mutual friends are around during the holidays and that you’re thinking of inviting them .
My mother was the first of her girlfriends to be widowed and she found she got left out of couples things .

Personal if you have always asked her as a couple why would it make a difference if she is on her own. Why not ask her and give her the opportunity to accept or reject. It’s going to be painful for her as it is being her first Christmas without him. At least she knows you have through about her. You could say I’ve thinking of invited such and such.