If the 85-year old unmarried couple are living together but not having sex, I don't see any issue. I hear that is not uncommon for people of an advanced age to want someone for companionship not sex. It is possible that they have deep friendship/connection and no sex, just like a 85-year old married couple.
If the 85-year old unmarried couple are living together but having sex, that is obviously not Biblical. If they want to have eachother's company and not staying together just for financial reasons, that's their business and adult decision. I would not get involved as long as they are mentally sound, except for telling them it's wrong.
If they want to live on their own but want help or want to live with me, I would help out. However, it is not uncommon for the elderly to want their own independence as long as they can. Eitherway, it is their own decision how they want to live.