How do I tell my elderly mother she smells?

It sounds awful I know, but my 84yr old mother has bad arthritis and because she is in pain,she has stopped taking care of herself. She refuses to allow anyone to help her seeing this as a way to hold onto her independence. Yet , this is not good as she no longer washes or showers, and her hair is dirty and unkempt also. I have offered to bring her to the hairdressers for a wash and cut, but she won't budge from her chair (her life is chair to bed, bed to chair, TV and nothing else , my dad does all cleaning and cooking and he is 80 ) She has now gotten herself to a point where she is actually smelly and I feel like I should tell her. But how ?