Red-1
I used to be decisive, now I'm not so sure...
That's a bit of a "I know it when I see it" question, and as has been demonstrated on this thread that varies for every person. I have witnessed some truly awful things done to horses (and other animals), and left jobs because of them. Some of the most abusive things I've seen have been caused by ignorance.
It's a twig for goodness sake, probably less robust than any riding whip. I fail to see your logic here though: whip OK; twig bad. Losing one's temper with a horse is never an acceptable thing, although in this instance I feel it's more with the rider (not an excuse).
It is kind of set down officially too though. A long time since I was eventing (2014) but I imagine the rule is still a maximum of 3 hits per fence? From memory, it was up to 3 hits, not at the majority of fences, and no marks to be left. Oh, and it would have to be with a whip as opposed to a branch off a tree. These days, I believe it also has to be a padded whip, of a certain length.
In no case would hitting from the ground be acceptable, whilst a rider was on board.
I also wouldn't slate the rider for her riding. Yes, she was sitting a little passenger-like. However, she was there to learn. The horse and rider were going sweetly enough together. The horse wasn't confident. With the combined level of skill and confidence, it would have taken a while to be confident, as in, not just the one session. They could have built the confidence together. They may well always have been hesitant, until both the horse and the rider became committed to go. They may never have got into new water down a step, but that would have been their learning path. Walloping the horse with a branch is not a satisfactory answer. I also don't think it would help accelerate the learning process as I don't think it would make the horse more confident to approach water the next time, wouldn't have made the rider more effective and he can't always be there.
It disturbed me that the rider, although now slating Mark Todd (as he has now admitted it is him), saw fit at the time to set the video to music, put some funny comment on and upload it to her channel. Although she now says it was awful of him (or words to that effect) it seems, on the face of it, that she thought it was light hearted. Or maybe that is a generational thing.