Dear Shilpi,
Such people are everywhere, almost in all organizations. We all know that every human being is unique and they possess some unique qualities to maintain their uniqueness. Some possess widely accepted unique qualities and some possess still-to-be-improved qualities. So, first thing we should understand is that this is a common phenomena and not exclusive at your organization.
This is where the HR role comes into play. Speak to her about the long years of contribution she delivered towards the organization. Highlight her achievements in the organization and how did they benefit. Then tell her the general feedback about her behaviors in the office. Update her about what people around are thinking about her. Tell her that people are highly impressed on her achievements but everything is getting wasted because of these minor behavioral deficiencies. Update her about the generally accepted behavior patterns in an office atmosphere. Tell her that if she improves her behavior, it is going to help her in a big way in establishing herself in the organization.
Some employees, especially when they are in an organization for a long time, create a critical space for themselves in the organization. Also, eccentric behavior is considered to be one of the qualities or remarkable employees. They are never satisfied with the way things are happening. They forget their roles and responsibilities and jumps in when things are not going the right way, wherever in the organization. In such situations, the management may behave in a lenient way towards their actions. This is quite common. However, when it goes out of proportions, someone should act upon. If the management believes that one individual is more important than the entire office environment, then none can help. This does not mean, HR Function should keep quite. We are the custodians of people and we should be thinking about ways and means to improve the situation.
MK Saseedharan