Devil's Den was the remains of a long barrow - the mound destroyed and the stones fallen. It was restored in the early part of the 20th century by a well meaning antiquary who restored to how he thought it should look. I was puzzled as to why it was the only 'portal tomb' in Wiltshire until reading the superb little book by Peter Fowler and Ian Blackwell An English Countryside Restored - The Land of Lettice Sweetapple (found in my local Tourist Information Centre and still one of the most useful reference books on the Fyfield/Overton area there is).
I just wondered if by reconstruction you meant it was "recreated" using non-original stones and did climbers over the ages cause the original knocking down. I would've thought that the renovators would've noticed rock-art especially if it is such a rarity in the area. BTW you should've voiced your opinion at the "disrespect" at the time even though you didn't feel it was harming the (reconstructed)stones, your associates might've had a little respect for your feelings and not investigated the stone quite as thoroughly in your presence.
In 'defence' of June, although she certainly doesn't need me to stand up for her, it's not always easy to voice an opinion when in the company of people who you may consider your peers or you are fearful of if strangers. A 'time and a place' often decides these things and I've been in that position myself on more than one occasion.