In Pokemon Diamond, Pearl, and Platinum,(for the nintedo DS) Cynthia's grandmother can be found in celestic town, which is located on next to mount cornet (it should be on your map).
I believe that her grand mother is also the elder of the town if that helps you.
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In Celestic town when you defeat the galactic grunt and give the old charm to cynthias grandma head into the ruins and talk to the picture of azelf,uxie and mesprit.Cynthias grandma will give you hm03 then.
December
5'7
In Celestic Town but i can't figure out how to get there .. can anyone help me with that?
She is in Celestic town. Fight the Galactic Grunt protecting the ruins in Celestic Town and Cynthia's grandmother will appear.