I’m sure this is a very rare and extraordinary occurrence. The video shows this monk who look Orthodox by appearance, and we are told that he is from Mt. Athos and that he wanted to spend a year at the Carthusian monastery. This is all the information I have, and so we do not know from this video whether he left Mt. Athos with the blessing of his abbot and spiritual father or not. It states on the video that he was interested in the solitude of the Carthusians, and yet there is certainly solitude to be found on Mt. Athos. It would be interesting to know more about this story, but I would think most Orthodox would consider it very strange and questionable for an Athonite monk to spend such a period of time in a Carthusian monastery where he cannot receive the Mysteries.