Dad has pneumonia but nursing home has him dnr. should it be treated?

Different issues: A dnr order refers to resuscitation during cardiac arrest and has nothing to do with whether to treat pneumonia. It's reasonable to treat pneumonia despite a dnr order. But in some cases, patients or their families may choose not treat pneumonia if it's felt that it would not improve quality of life. Pneumonia used to be called "the old man's friend" because it was a fairly benign way to die.

The end stage: When the end comes near there are some basic ideas that most are comfortable with. We all recognize air hunger and thirst as painful and would never withhold oxygen/fluids. The difficulty for many is embracing the idea that you should not prolong the act of dying. If your dad is comfortable, this may be his friend, if painful (to him) treat.