1) Be aware of your genre blind spots.

If you read, or write, a specific genre make sure you do not expect pieces of that genre from the author you are mentoring.

Avoid asking someone writing a contemplative short story about moonlight "where the adventure" is. Resist suggesting a cutesy chick lit story could benefit from some zombies or vampires. Refrain from attempting to shoehorn a "hot love interest" into a gritty, police drama. Don't say you aren't understanding the "rules of this planet" when your author is writing about a typical school boy at a typical grammar school. 

Once again, it's NOT your story. You are navigating, not at the wheel.

2) You don't have to like your author's story, you just have to make sure it's quality. 

Not all stories you'll like. They may not appeal to you or be up your tree of choice. They don't have to be. That's not the point. The point is making the story good, no matter what it is. Quality is the name of the game. Resist the temptation to steer your author away from what they want to write about to what you want to read about. 

3) Your preferences are irrelevant. It's the author's vision, working style and preferences that count.

We all have our preferences. Some of us like to write everything out and then show our work, others like to have someone stand next to them, read and talk to them about each scene they've written. Some people need a cattle prod, others will send you messages every day. You have to be flexible enough to accommodate all sorts of different styles of working, not just your own. 

As far as writing goes, some write in stark, bare prose, others in flowery flourishes. Fine. The only reason a mentor should suggest a change of style is when it does not fit the story, or actually detracts from it. Otherwise, what writing style you prefer to read, be it Hemingway or Wordsworth, makes no damn difference to the task at hand.

4) A mentor is not a critic

Remember the difference between a critic and a mentor? A critic simply says what works, what doesn't, and gives a thumbs up or a thumbs down. If all you're doing is reading through your author's work and giving a summary of your impressions plus a star...you ARE NOT mentoring. You are critiquing or Beta reading.

Stop. Rethink. And start again correctly.