I'm not in a diplomatic mood today but I'll try to add my opinion without offending.

I agree with PSMKay. Maybe it's because I've dabbled in education, but I have a difficult time understanding why someone
wouldn't want to share their knowledge. And to state that your reason is because you don't want to equip someone with the tools to become your competition is showing that 1. you think they're capable of doing so, 2. you're at least a little unsure that your combined abilities aren't substantial enough to get you jobs or 3. you think you're holding the Holy Grail of stage management, which, to speak frankly, sounds pompous.
Imagine if the greats in other industries felt the same way your friend did, Matt. Where would those industries be? How would the world of cooking be if Julia Child didn't publish Mastering the Art of French Cooking and just kept it to herself? Where would actors be without Stanislavski and Kazan sharing what they've learned to the next generation of performers? If the masters of music composition didn't take on students, if the masters of the art world didn't take on apprentices...the world would be quite different. They taught others so that when they were done their students could take it to the next level and improve their industry as a whole.
The other side to that coin is that be careful who you share with. Someone who is on your same level, who is constantly on the same short lists as you for the same jobs, who walks in the same tight circles and knows the same people...of course there's a real threat there. But someone who's still new to the whole thing, who may not be in the same field in five, ten years down the road, who may decide not to join the union, who may work in another part of the US, who may go work in Indonesia instead, or join the circus... The world of stage management is not limited to Broadway. There are jobs in other places around the world, and the weak ones will give way to the strong ones. At least, you certainly hope so!
Authors in the art of selling your brand will tell you the same thing: show yourself as an expert in your field and the jobs will come to you. They also say that to show yourself as an expert, give a part of yourself for free: teach classes, give lectures, write articles, make speeches, mentor students. No one will know you're an expert if you're huddled in a corner with your toy yelling, "Mine! Mine!". So to speak.
The beauty of what we do is that no two people do everything the same way: paperwork, blocking, handling problems, talking with actors, etc. We all do it differently. Someone may be getting advice from 20 people but may only follow the advice of 3. Why? Because the way they interpret the advice, if it suits their style, if it suits their personality, all depend on what advice they gravitate towards
Yes, I've also noticed the influx of high school and college students into the forum. The ratio of students brand new to the field to those that have been plugging at it for a large part of their lives is growing. But by the time those students are at the level you are at right now, you're likely to be gone, either out of the industry or in a more permanent sense. Knowledge gained from experience, not book knowledge, is what makes a stage manager an expert in the field. To think that a senior fresh out of college is ready to compete with you who has been around the block (hell, built the block) is going to be a real threat of competition is being narrow-minded.
That's not to say you should give up
everything you know. I mean, c'mon, it's one thing to give you the recipe. Quite another to withhold that secret award-winning ingredient. If they're a good enough cook they'll stumble on it on their own anyway.
I hope that all came across somewhat coherently.