Poor thing. She may not want to discuss her problems with colleagues (even a mentor) for fear of appearing incompetent. In any case, mentors usually have too much on their own plates to be able to give much time to an NQT. I suspect it is difficult kids who are giving her the most trouble. No doubt the school has a policy for dealing with bad behaviour. It will be something like yellow card/red card/parents informed but whatever it is she needs to find out and try and make it work for her. Teaching is really hard work, no wonder we are short of good teachers. I think all you can do is listen to her problems, just let her talk, and then try and encourage her to at least complete her first year. It will get easier for her as she becomes more experienced but in the end it may not be the career for her.