Um, did it occur to you that you could offer to push the button for that person right away, making the need for them to be anywhere near a button obsolete? Maybe it all happened too fast for that. I rarely hear people ask for help with something like that because they are too shy or don't want to bother others. I often hold the door and ask others what floor they want to help avoid situations just like that.
I do the same thing, if I'm the first one on, today I was 5th. I'll frequently hit the button for my floor and ask people where they are headed, or I continue further into the elevator to make room for others. But I can't offer to help someone when they are first on the elevator and have successfully blocked the entire doorway to the elevator (still not sure how one person managed to block a space 6 feet across, but it happened). I don't know if its because our building is currently occupied by only a single company (or collectively we're very lazy and most people have gotten pretty good at elevator etiquette), but I regularly hear people asking others to press the button for a certain floor for them.
My point is what happened today was remarkable for how the elevator experience typically goes, and that is annoying.