I do not disagree with you JBF but his mother should have known better. As I said, it his mother's job to appreciate his choice but explain to him why it is inappropriate and help him choose something more appropriate.
Adult sexuality is on the media, whether as books, newspapers or online so of course children will come up against things that we would all think they should ideally not know of. But our role as adults is to guide, protect and contextualise the things children see and read and that includes explaining why Christian Grey is not a suitable character to dress up as for World Book day at school.