I think I live in that sort of house and I think you would feel the same if you visited me - unless I know you were coming a few days in advance and had time to do some extra tidying up!
My perspective is, I would like help but it is very difficult to get non-judgemental help.
And where do you get it? Social Service help has been so heavily cut in the last years that your friend probably would not qualify. Getting a private cleaner is very hard. Friends want you to 'get better', but you can't always.
I think she probably, just like me, needs help from someone who just accepts who she is but most people try to encourage you to - walk more, just do 10 minutes tidying up a day, volunteer!!! Oh the list goes on and on of what people think you should do!
It's hard to get out of this situation, if she wants to. If she is very well off, probably she could pay an agency to clean the house regularly. Otherwise, difficult.
I have enough people, neighbours etc who don't hesitate to tell me what needs to be done. I know it does in their eyes. But what gives anyone the right to comment on how I live (though my upbringing etc. absolutely silently agrees 'I must conform'!
But, also, self neglect can be a serious medical sign of depression. loneliness.
At least she let you into the house. I know many people
would just make an excuse not to let you in.
Is it just her house or also garden - things you know she cares about. Is she still interested in the things she always was? Does she do anything all day or just sit about?
Maybe she doesn't see just herself as deserving to live well.
'It's just me, it doesn't matter' is how many of us feel.
It can be temporary, stay as it is or could get worse. From what you say, it doesn't sound too bad tbh.
Maybe just go, have a regular coffee - at a cafe would also be good occasionally, to get her out if she goes out less than before? Ignore the mess. Show her you value her for herself, she may not be feeling very valued at the moment.