400 meter off shore is not far for a kiter if working upwind to get back to some good waves.
If both of you had been holding the same direction for some time and you guys just happen to meet, providing he past behind you and on the down wind side then there is very little chance of anything bad happening. Perhaps he was even being courteous by changing coarse slightly to actually pass behind you.
If he passed in front of you and upwind, well yah that is stupid.
If he passed 1m from you to do a trick 400 meters offshore call him what ever you want

A for effort on the photo collage