As long as it is clearly communicated by him what he's doing and why, any approach is fine really. So as long as he tells her his exact purpose of the break and what he needs to stop it, all good.
If that is not done, and it's just a one-sided decision of his to stop talking, not even explaining anything, then it is very bad. It'd basically be like a parent punishing their child and not telling them what for, mentally very problematic. Of course it should be able to be implied by her in some way, but it's very easy to come to the wrong conclusions.
Second question, you simply don't let them. You calmly keep repeating your question, pointing out their intentional ignoring, stating that you will only talk with them about anything else after this question is answered, until they either get so mad as to run away, or they respond to it. But you have to actually stay strong, and not allow them to do it. Depending on other things that need to be communicated, and how stubborn they are, that will hurt yourself as well as them since other important stuff for you might not get communicated. But that is something that needs to be tolerated, because breaking from the original intention is worse for the future, it shows that ignoring the question works, and they'll repeat it.
Oh and as for reasoning why, another few points, all projects I've been in just kept being worked on and had constantly changing requirements. There was no real need to plan very much except maybe some rough estimations, that were allowed to be wrong.
There were like some very rough aspects of scrum in professional development, but only in the sense that we'd talk about what we'd like to do in the next sprint, we didn't do multiple plannings or estimations or cared about our velocity or did retrospectives often, and even the sprints were adjusted to last longer or shorter based on what we're going to do, there were a couple of roles people should have missing, and idk what. In the end, the resulting system was just something in the direction of agile/kanban, work just came in, and was handled based on some prioritizing by someone.
My personal projects could be really close to waterfall as well, I thought about a problem, made a rough plan on how to solve it, then just kept solving until I was done. Open source projects, no one organized anything, everyone just works on whatever they like.
Basically, you're the expert in software development paradigms, I'm just a developer that works on problems with code until solved, either given to me by someone or myself. The only ones who care about the paradigm are the business guys who wanna plan some shit.