Worth testing elsewhere first , if he was willing .
If he can show that he is capable of finding anything that nobody would be expected to find without dowsing then it's worth testing at Stonehenge .
But I doubt that he would be up for a test .

Anyone could go to Stonehenge with or without knowledge of the site and suggest that there are pits , cremations , buried stones , Anglo Saxon coins ,Victorian bottle tops and there is a possibility they might be right or close .If a dowser did the same it doesn't mean it was due to dowsing . Finding previously unrecorded archaeology in an area where none might be expected is done all the time by non dowsers and non detectorists on a daily basis .If dowsers could show , even that ability they might get accepted and it would be a start ,but they haven't