Probably tell your friends when they make big mistakes

[unsure whether to up- or down-vote. This seems too high-level and over-general on a topic where details and nuance can easily override far-mode preferences.  But it's important and feedback on errors is one of the best ways to improve one's rationality (on any dimension of error - modeling, weighting, implementation). ]

You imply in your closing (which I like a lot - it's a concrete request and offer) that this feedback should be private.  I wish you'd mentioned that sooner, and acknowledged that it may be happening more than you think, since you won't see most of the feedback to others.  

I also think that's a little at odds with the type of feedback that should be given to larger projects or ambitious people trying to have a large impact and failing.  This feedback should be BOTH public and private.  The ways they're doing harm should be available to the people being impacted, not just to the actors.  The distinction between individual decisions and organizational or group (small or large) decisions is very important here too - there are different methods of giving different dimensions of feedback, and being good at one is almost unrelated to the other.

Finally, I wonder what purpose you had in specifying "your friends" in the title.  I do a lot of 1:1 discussions and performance reviews with software engineers - giving and getting feedback on work topics is a big part of how I improve, and how I improve my teams.  I try to give (and seek) advice and evaluation in most aspects of life, but "my friends" is probably the group where I'm least focused on improvement through this channel (meta-discussion about how to improve), and more focused on fun and improvement by object-level trial-and-error (new activities, games, topics of conversation).