I run into this problem all the time and from both sides. Someone I'm skiing with thinks "this guy is better than me" and on account of that chooses not to say anything while I'm skiing even if they notice something that looks funky. Maybe I'm alone here but if something looks funky, I'd prefer to know. Ok, maybe they can't work it back to the root of the problem but if they notice something that turns out to be a symptom then with any luck, I can work back to the cause.
I have been in a slump most of the season, missing passes I simply don't miss. Today, a skier I was with told me I was dipping my shoulder in at the turn and swiping the handle across my chest. Both accurate, both symptoms, and both things I wasn't aware I was doing. Because he mentioned those things I was able to attempt to address them head on and was able to work back to what the underlying cause was.
So maybe others will disagree but if any of you ever ski with me and think something looks funky please don't hesitate to mention it, yes, you are allowed to coach me.