Doesn't it depend on the type of session? If it is an open, public session (illegal now in England I hear...) You really sort of have to allow anything. And , truly, people don't get better at what they do unless they are allowed to do it in context. If it's really excruciating, some good natured and thoughtful criticism can go a long way towards developing someone into a good sensitive player (as opposed to blasting them, embarrassing them in front of everyone and demoralizing them for life). I mean, what is a session for, anyway? Not EVERYONE can be as good as YOU, eh?