My heavens! I hadn't heard that term used in a long time! Back in the '40's and '50's, when many people regarded divorce as a disgrace of sorts and spoke of it in hushed tones, around the children they often used the term "grass widow." Now that divorce has become so commonplace, the term has become more or less obsolete. I would guess, being a retired English teacher, that the "grass" reference is to the fact that one or the other, or both, of the marriage partners found the grass greener on the other side of the fence! It could also, I suppose, easily be used to describe desertion of a spouse for the same reason.