I think there is so little money in writing that it would be a crazy or very inexperienced writer who would agree to work for a percentage. I suggest you rather buy a book on writing, read a few good books that are similar to your friend's story, and try your own hand at it. Once you've written 20 pages, finding a good editor would be a lot easier, and cost much less, and a good editor would help you bring the project to fruition.