Dear Nigha
Re your question as to whether HR personnel should inform the boss if they have information about any staff planning to leave the organization.
My comment would be YES as I feel that it is our responsibility to act in the best interest of the company. However, before we give out the information, we need to find out the reasons that has caused him to think of leaving the company. Remember that 'People join good companies and leave bad bosses!' or they will move on if they feel that there is no more opportunity for both personal or career growth in the company.
Having knowledge of the reason that has caused him to plan to leave, it is then our responsibility to work out a plan (which has to be aligned to the company's policy) to retain him if he is a valued employee and also, a succession plan if we know that we will not be able to retain him.
Just structure all your information properly and inform the boss accordingly i.e. reason for his planning to leave; feedback from superior and peers on his general performance as in terms of teamwork, productivity, etc.; pros/cons of losing the said employee; proposed retention package; succession plan; etc.
If we handle this subject objectively, the boss will understand and the position of the said employee will not be jeopardized.
Well, our goal is to achieve what is best for the company and it is very important that HR must play their role responsibly.
Best Regards
Catherine