My theory was that she might be more amenable to changing in the morning. Sometimes this worked in practice. I would lay out fresh clothes in the correct order (sequencing of tasks was now difficult for her) and persuade her to give me the clothes that she had slept in. I had to place them out of sight straightaway otherwise she would put them back on. On better days I could get her to have a wash of sorts before putting on her fresh clothes. On other days she would be 'ready' to take the dog out wearing nothing but her coat.
Things improved after I engaged a carer/befriender to help 'look after the dog' when I was out. She could usually manage to get my wife to change her clothes and sometimes even to have a bath. It was only once a week but better than going weeks on end without washing properly or getting changed.
This guide might be helpful:
How does dementia affect washing and dressing?
Find out why washing and dressing can become difficult for people with dementia, and get practical tips to help with washing, bathing, dressing and personal grooming.