I am sorry your son is going through this. I am sure everyone does not mean to place all the blame on the DIL. Having been through a messy divorce its never only one sided.
I say take a day or two and get your mind heart and spirit settled then work on the quilt with lots of prayers while working. I would even tell her your working on her quilt and praying for the two of them.
What better way to show unconditional love. How could she not make sure her children see their grandmother who showed such love during a very bad time? You never know this could be a cry for help from her......