My suggestion would be if you find an error with a Find A Grave Memorial, find out on that page who the creator of the memorial is and see if they have a link that you can contact them with. There have been a few instances where I needed to have a correction made on FAG, and with the help of the page creator, we managed to get the correct information on the memorial, whether it be date of birth, error on the death date, error concerning the listed relatives etc. Some of those volunteers are also here on WikiTree and are most willing to assist.
Another recommendation is to ask the person to have a picture taken of the tombstone. They may have to ask around to see who handles that area but it has been done. On the contributor's profile page there is a link for photo requests in most cases. You may have to sign in to FAG but its free to join, and free to ask for assistance.
I wouldn't give up on "Find A Grave", again, they are volunteers just as we are here on WikiTree. Most welcome your emails and or suggestions on their contact page.