For any such mentor, you can say "forget about the bad, focus on highlighting what the person did right". This will almost always dramatically increase the mentee's rate of progress.
This is because for any given task there are a million ways to get things wrong, but only one/very few ways to get things right. So people learn almost nothing from "don't". You can eliminate negative feedback altogether and lose almost nothing.
Of course, it can be useful in very specific situations, and these are generally not cases you need to sugarcoat because they are very technical (e.g. "don't spike the ball when leading by more than 14 points", "don't close the issue when a manager has commented on it", etc).