What's he going to do? Trained professionals are taking care of the evacuation, and if he tries to "help", he will just get in the way. Sometimes there really is nothing you can do. This is not to be confused with a case where nobody is doing anything and everybody is just watching.
Exactly; unless you see a passenger struggling to get away from the plane, it's not at all clear what you could do early on without getting in the way or running an unreasonable risk.
Based on the early and very possibly incorrect or misleading info and pictures, if it were me I'd hang back and then volunteer wherever they set up a staging area for the less injured survivors.
