Excellent advice, and it applies well beyond the specific audience you have discussed in the video.
I can't emphasize enough that allowing for mistakes is one of the best ways for truly teachable moments to occur, and two things that get taught (if you do not do the "Don't do that!!" yell) are that this is a natural part of using a computer, and you can "work your way out" of whatever situation you get yourself in, whether gracefully, or even otherwise.
The only time I try to avoid allowing a mistake to happen is if I honestly believe, from observing during a lesson or lessons, that it is going to reinforce an unwanted behavior. Even then, you don't stop it with a declaration that causes alarm/nervousness, but try to get the person to pause, then ask them what they are trying to do, and what they were about to do next. If those two things are not compatible, you then explain why, in the simplest and most direct terms.