You don't actually have a legal relationship with the tenant, you, as a Coop/Board have a relationship with the shareholder of the shares related to that apartment. You therefore have to direct your efforts to dealing with the shareholder, who is answerable for and responsible for, ultimately, the behavior of their tenant(s). They are in a bad position given the rights of Rent Stabilized Tenants under the law, and the Court being very, and appropriately, protective of the disabled and seniors with possible dementia issues. Is there any family with responsibility here? A guardian? You might need to contact social welfare services etc. in the process. IF there is anything that is occurring that is a potential endangerment of that person to themselves or the other shareholders then you need to call the Police, or Fire Department, especially to get it on record. Do not enter the apartment at all, if there is an emergency and the Board has rights under those conditions, there must be at least one Board member to accompany the Super, and it is better if a Police Rep or Fire Dept Rep is with you, as they have legal protection that you do not. IF you enter without them and there is a problem, or question as to the 'realness' of the emergency, you can be in a lot of trouble.
The most important thing is to start a paper trail.EVERYTHING in writing, return receipt, and consistent, do not let time lapse as the Court will view it as not current or just occasional. Your best shot is to leverage the holder of unsold shares who has no protection from you writing and filing for breach of Prop Lease, Bylaws, House Rules, whatever is affected. You must keep an accurate log of times, dates, incidents and who reported to. Be proactive, issues do not usually go away by themselves, they tend to progress and deteriorate over time. Be kind, but be firm. You have an entire Coop to protect. It is best to involve your Coop attorney early in the process if you feel it is not going to be a simple matter.