P.G.
Is grandfather moving way far away, or just not living with grandmom? That would make a difference in the explanation. I'm not sure what to say, but maybe just be very simple in your answer - Grandma and grandpa don't live in the same house anymore. It might be enough for her for now, especially if she can still see both of them at their own homes. Hopefully they are on good enough terms to put aside their differences for birthdays, holidays, etc. By being the grownups and continuing to honor their relationship with her, I would hope it would make things a little easier for her to deal with, and more pleasant for them. Good luck!