No, the divinity of the Salagrama deities will never diminish because your grandmother had touched them. But women do not touch the deities as per the custom. Elders handle the deities only after performing the appropriate purification rituals. They do thirumanjanam with milk for purifying the deities.
Women should not take this incident as precedence to touch the deities. Women can worship the Salagrama deities mentally and offer food in the same manner. Women can clean the place around the Salagrama box, decorate it with kolam and flowers, light up lamps but never touch the deities and perform thiruvaradhanam.