If he's purchased the article and written exclusively for him then he has the rights to it and can use it where he wants. If he's only used it here then this shouldn't be an issue. As I've seen plenty of times. If he uses it on another site where he owns access to like a blog he just needs to post on there indicating that he also posts on Steemit to show he's the owner of the other property.
there is one other question that comes to mind here.. if this was written for you, how did anyone even know the difference? did some of the content show up elsewhere?