Well as you know already I have been doing Chi Kung for over 7 years.
I have experienced many different feelings and things I cannot explain in this period of time. Miraculous healing, controlling heart rate, heating or cooling my body at will to a certain extent. Having people go dizzt standing next to me on occassions etc.
Anything is possble / plausible as we all know. Proving it is another thing entirely.
I would suggest it may be the lactic acid build up, I dont necessarily get shaking with it, if you hold any pose long enough your lactic acid will deteriorate rapidly and cause muscle shaking, now if it was just a gentle swaying of the body, or a feeling of the body tingling and spontaneaously heating or cooling in the extremeties I may be able to say yes it could well be. http://www.wikihow.com/Do-Tai-Chi%27s-Horse-Stance
I believe this is the pose your friend was told to do.
One of the most basic stances when you get the posture right, good for core conditioning also.
Question I would be asking though is
" how do I know what the chi feels like if I have never experienced it "
" once I do feel something how do I know its not just my body being tired and the shaking is not the energy but my body wanting to say time out, pack in, I have had enough ".
I would think and its only a supposition, that teaching Tai Chi scientifically would be harder to do, rather than just getting on with and explaining where it comes from, what the benefits are and letting the person make their own mind up with regards to what they get out of it for themselves. I think gathering Chi is the last thing people want in a class as most would probably not know much about it, but they may have seen the health benefits from tv, magazines, books and other advertising, indeed may have even done some basics in a martial arts class.
How can you explain something which can vary from person to person all over the world?
How can you explain the fact away that just because your body shakes it is the chi making it happen?
I hope your friend has the sense to question and ask questions in class to get to the answers he wishes to find, I would bet that the instructor is not so open to receiving questions and being questioned also. Call it a hunch.
Anyway, I think I have rambled on long enough.
Just my two cents as always