This is really sad, and I wish the guy a speedy and full recovery.
Even more sad is the fact that I've seen exactly the same thing happen at Sandbanks before. About 3 years ago I witnessed a young guy, probably around 17, hot-launch his foil kite in about 20-25 knots of wind directly downwind of himself. Even more frightening was the fact that his kite was on the Northern end of the sandy 'beach' in the harbour, meaning that he only had to be dragged/lofted about 5 meters before he reached the low boundary wall and then the road.
As it happens, he was dragged onto and across the road, across the path of (luckily slowly) oncoming vehicles, after which his kite got tangled in a tree and he was able to release it. He was lucky and escaped with minor scrapes, but it could have ended so much worse as he wasn't wearing a helmet.
Wake up people!