I'm so sorry to hear this for all of you, I feel your grief as I've been through it and know the searing pain brought on by seeing your adult children's family break apart. It won't help just now, but the searing does eventually ease back to more of a dull ache.
The thing now is maintaining your relationship with your grandchild - it's not easy (sadly I know this only too well) but it truly is a must-do. My son and his wife split when DGS1 was 2, he's 13 now, and while his parents go back-forth regarding reconciliation (they were on-ish but just before Christmas have become off again - this will be yet another 'rinse and repeat', sigh), DGS1 told me Christmas Eve my determined remaining important in his life has made all the difference for him - he knows no matter what he can count on Gran to not take any side but his.
Keeping close to him has been difficult especially as I live in Scotland and he lives in Alabama USA but between snail-mail, email, Skype, and weekly phone visits, we manage - I communicate more with DGS1 than I do his parents and I don't feel the slightest twinge as I've believed since 2004 the boy is who is most important.
My DIL and I actually have a good relationship, and of course my son and I are close - and both say my being the one steady for DGS1 is a great help.