Consider this analogy:

Two people are in a car. The owner of the car is in the driver's seat. Their hands are on the steering wheel and they are always in control of where the car goes. The other is in the passenger seat with a map in hand. The navigator gives the driver tips and instructions about how to get to their common destination.

If you haven't guessed, the author is the one at the wheel and the mentor is the one navigating. 

It is the author's responsibility to drive the story. That is, to actually WRITE it. Not the mentor's. No matter how much you as a mentor might want to rip the wheel out of the hands of an author to stop them driving the story straight into a ditch or a brick wall, you shouldn't! No matter how much you as a mentor think you could write the story better than the author could, resist! It's not your car. You don't have license to drive it. Keep your hands off the wheel. 

What is your job as a navigator, then? 

Simple. You are the one with the knowledge of the road and the ability to get the story to its correct destination. That's right: knowledge and knowing the road to the goal. 

What type of person makes a good mentor?

Since you are the one holding the map, you need an ability to logically plan a route. That means you need to be able to strategize and plot. You will need to be able to figure out how to get from A to B when the author has stopped the car at a busy intersection and doesn't know which way to turn. You need to have good, applicable ideas, and be able to see around and through thorny thickets of problems. 

And, just as importantly, you need to allow the author to drive at their own speed and in their own style, making all of their own mistakes and learning how not to make them again. Remember, the point of making mistakes is to learn from them. No mistakes, no learning. 

Thirdly, the more you know, about everything and anything really, the better mentor you will be.

You don't have to have any fancy degrees or have attended any top schools, but you should have a fairly good general knowledge in the humanities (history, mythology, sociology, art, literature, cultures, languages etc). I won't mention here that a good knowledge of the craft of writing is essential.  (I would assume you know that already)

It's this ability to see what's on the side of the road, where a slight detour might be beneficial and where the 24-hour bagel shack is, that makes for a successful journey to a finished piece of literature.   

A good navigator is a good mentor.