It's tough GrandmaH and your DIL needs to feel she hasn't alienated you in this process - it's so important to stay as friendly and supportive as you can, if she will let you (my ex-DIL wanted open hostility). Keep doing as many normal things as you can with your grandchildren and if you are able to keep contact with her side of the family and remain friendly, that really does help.
If and when another relationship develops, there's a fine balance to tread, being welcoming to the new person and not making them feel you prefer the first partner, but also remembering the first partner might feel ousted if you are friendly to the new person - bit of a minefield!
I have found with my daughter and her ex that he has wanted to distance himself, but I always remember his birthday and send joint Christmas presents for him and the children for when they are staying with him, and I send him supportive texts, even when I know he's being difficult about shared parenting. I don't intervene, but try to be helpful in the hope he will have a conscience and improve. Her new partner sees me being friendly towards the ex, and I have said a couple of times to him that I want things to remain on a friendly footing, but this doesn't mean I am cosying up to the ex - her new partner of a year is delightful and much more suitable in my opinion, and I hope he feels secure about how we relate to each other. As a couple they have invited her ex round to BBQs and the children's birthday parties, but he has declined so far - they will keep inviting him.
Having been in two different situations where there has been friendliness and hostility, I've found the obvious - the children are happier and able to be themselves when they don't have to take sides and can see their parents coming together in their best interests. The first year after such separations isn't usually a good gauge of how things are going to be, as both parties will move on and the split won't be as new and raw. Good luck