How do I convince my Dad to allow in-home help with Mom?

My 92 year old mom and 81 year old dad live several states away from me. Over the last several years, mom has shown increasing signs of dementia (although her doctor has not used that diagnosis). Her memory is significantly altered, she is frequently angry (at the news, at Dad, at strangers), and has become increasingly resistant to going out of the house. This means Dad -- who has become her care giver -- has become isolated too. Mom fights / resists his efforts to have a caregiver come in a few hours a month while he goes to appointments or just to get out of the house. I know it is unhelpful to reason with her, but Dad needs the help. How should I advise him? Mom takes out a lot of her anger on him, and he's stressed and frustrated, but doesn't want to ignore her wishes. Help and advice would be so very much appreciated.