I had a quick look on the internet and found an article (with photos) on "grandmother of the bride dresses" on www.dressforthewedding.com. I liked all the outfits shown. There was a navy blue guipure lace and chiffon pantsuit with jacket, which looked lovely. (Just checked it's no longer available - but useful for ideas).
David's Bridal have some good articles on their website, including one about choosing colour of outfit to go with the colours worn by the others in the bridal group. I think blue or grey, or blue-grey, would look good. But it depends on your mom's preferences, and what will blend with the bridal group. I hope she finds something she likes, and has a lovely day.