My mom is crying and grieving the loss of her mother and father, who both died 50 years or so ago. How can I help her in her grief?

I agree with the suggestions below. Help her learn to celebrate their lives by making notes, collages, scrapbooks of memories. This may help.

I agree with what Beatty said below. I would even suggest if you have old photo albums of her parents and family, to sit and go through them with her as well. And allow her to grieve if she wants or needs to. And I loved the suggestion to set up a little memorial of her parents,(using their photo) that she can look at every day. That might be all it takes to calm her down.

Oh that's so sad.
Talk about love. How they loved her. Make a connection to them? Is there a nice photo? Everyday day talk lovingly to the photo. Lay a flower from the garden in a case there. Blow them a kiss. Have a hug. Tell her they are happy in heaven (or your version).