I have an OH with PD, and my mother had Lewy Body Disease - a particularly nasty variant of PD with dementia - so it is a subject close to my heart.
Your poor Mum could indeed have depression or the beginning of dementia. One thing that happens to people with PD is that they gradually become more anxious and their lives close in - and their partner is dragged into that whether they wish it or not. Equally, ceasing to care about her appearance could be an early sign of dementia, but in the absence of any other symptoms it may well not be.
Can you discuss it with their GP? - it is not going behind their back, but simply trying to clarify what you might best be able to do to help them, and expressing your concerns. She/he will be very used to that sort of discussion.
This is a very difficult situation - I used to be a SW with elderly people for pat of my career and know the scenario well. Sometimes it is just not possible to intervene if the elderly person does not wish for help. But it is important to establish whether she has a clinical depression that might be treatable first. I do think the GP would be a good first port of call. Good luck with this very difficult situation.
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