Great judgment! This is something we all can practice and keep practicing. It is a very important skill to see when situations with horses might turn bad. It's not just beginners who need to watch for this type of situation, but everyone. We can use this skill to not enter a warm up pen at a show with a horse that is panicking, to not go on a group ride with people who are being reckless and ignoring basic safety, to stop riding forward down a trail with hazards or out-of-control dogs.
We also should always respect any rider, experienced or beginner who has reservations about anything. Sometimes a person might just need a little pep talk, but as my friend says, "everyone is allowed to have a bad seat day." If someone does not feel confident for any reason, that is perfectly all right in my book. It can be a good day to do something different.
I always tell people I'm riding with, "Tell me no. Speak up if you want to stop. Don't do anything you're not ready for." A good philosophy is "ride to the ability of the weakest horse or rider."