Don't have any advice, but as with any bereavement, especially loss of a child, I'm so sorry 
Sometimes people just want something tangible to wear to remind them of their loved one, when my mother died I took to wearing her wedding ring on a chain round my neck for a while, just touching it reminded me of her. I guess your son's girlfriend wanted such a keepsake, I suppose that's understandable hopefully this will end up with your granddaughter one day. Nevertheless, sorry for the additional stress at this sad time, as others have said, it would be best to try and have a cordial relationship with the mother of your grandchild, as you are the strongest link to her father and can imbue her with the essence of him once she is older.