I was once in a situation similar to yours. The author acknowledged the error and in principle agreed to publish an erratum, but it gradually became clear that the author would probably never get around to doing so. I eventually resolved the issue by offering to act as the author's secretary, TeXing up the erratum myself but putting only the author's name on it. The author accepted my offer and everything went according to plan. The published erratum bears no sign of my involvement and it's conceivable to me that the journal to this day thinks that I really was just the author's secretary and not another mathematician. In my mind this was nearly a perfect solution. Of course I got no credit but I didn't want any.