As I said in my comment above, it is probably more appropriate to acknowledge the editor in the preface or acknowledgments section of the book. Usually when an editor is credited on the cover, it's because there are multiple authors of various chapters. This is sometimes called an edited book. With that kind of book, the editors are the ones who get the billing on the cover. For example, one of the most cited textbooks in the veterinary medical profession (the field in which I work) is Textbook of Veterinary Internal Medicine, edited by Drs. Stephen Ettinger and Edward Feldman. They are the ones listed on the cover, because there are dozens of contributing authors.
In your situation, there is one author and one editor. I think it would be unusual to list the editor on the cover. The author could be asked to write a paragraph to acknowledge the significant contributions of the editor. As an example, Theodore M. Bernstein, in his book The Careful Writer, included this particularly elegant acknowledgment in his preface:
Almost a collaborator, Bertram Lippman of the English Department of Bayside High School, New York City, shares credit for whatever is right in this book. Whatever is wrong is probably the result of my not having taken some suggestion of his or not having heeded some caution of his.
Also, I thought it worth mentioning that if you are applying for an ISBN for the book, you will be able to include both the author and editor names in the application. That will ensure that both people's names show up in searches for the book.