INVISIBLE BLOCKS?? (this is not a barrier block)

Once I was playing Minecraft with a friend using Discord voice chat. Then all of the sudden I saw my friend walk across a seemingly invisible block (he says it is end stone). We were in the End at that moment, and in survival on an server. I was like... umm... how did your walk over thin air. And he was like 'what do you mean?'. And I am a bit of a coward (due to the fear of losing netherite armor, and it only being a thin strip of blocks) so he broke them and placed them down again, and weirdly enough,  the End Stone appeared this time. I know he doesn't own the server without a doubt.