He is the only one who would do such a thing, in my opinion. One of his goals in life is to be of service to his "noble patroness." To be the one to inform/warn her of her nephew's impending nuptials, would be a real feather in his cap. He would feel he was really doing her a major service. I expect that his information came from Sir William or from Lady Lucas, who are very proud of their son-in-law particularly because of his connection with Lady Catherine, and would want to be the first to inform him. It is possible that he got the news from Charlotte, of course, who had received it from her parents, but, to me, it seems like the kind of thing that dear, self-important Sir William would do.