It is an oft-repeated suggestion, but we’ve not used it before here.
When visiting that cemetery, consider putting a waterproof calling card on the stone or near to it. A business card in a plastic bag, or a laminated one will work just fine. Use a stone, rock, or some other object to secure it in place, without harming the stone.
You never know when another relative, who doesn’t use the internet at all, might stop by that same cemetery and find your card with contact information.