Your friend is both smart and wise. I haven't even started a chainsaw to do a little carb adjustment or such without chaps or saw pants on in about 45 years. I'm that convinced that even the best of us can have a bad moment, and with a spinning saw chain a moment is all it takes to cause serious damage.
I will admit that seeing in person a couple of chainsaw cuts co-workers gave themselves in my early days as sawyer, was enough to scare me into this point of view. We have all seen close calls, I am sure. My USFS training clearly reinforced my actions in this.