What is substantive editing? I'm glad you asked. Substantive editing aids in developing plot and character development, dialog, pacing, and story elements, as well as story inconsistencies and some grammatical errors. Think of it as "the big picture" edit as opposed to a line editor or proofreader who only looks at grammar.
Mentoring is a little more involved than mere substantive editing. If the client's manuscript needs more work before it can be substantive or line edited, I would assist the author in developing and understanding story essentials.