Alas, some people choose to live in filth. Either because of mental illness or a complete lack of prioritizing a clean home. I’ve known both.
Growing up, my abusive, negligent, narcissistic mother loved living in chaos, cockroaches, and animal feces. And she subjected us, her kids, to living in it too until we were old enough to start cleaning up. She was flat out lazy. She preferred to let the gross fester until one of us caved and dealt with it. We had to throw away furniture because of infestations. Had to tear out all of the carpets in the house because of cat sh*t. I stopped eating cereal after I found bugs floating in my milk. She’d bring home countless stray animals and let them run amuck; urinating and defecating wherever and leaving it for us to clean up after. The fleas and cockroaches were unreal.
She enjoyed adding to the filth and chaos to stress us out further. Also, it discouraged us from bringing friends over. Abusers like her prefer to keep abuse behind closed doors without witnesses. Though, even the people who knew did nothing to help us. She never cooked, never cleaned, hardly supplied food or clothing, and unless we bought it, she didn’t provide supplies to do any of it either. The only time she did my laundry, was to purposefully ruin, stain, and shrink everything I owned (tossing in orange fabric, bleach, and washing and drying everything on high heat).
The only reason she doesn’t live in filth now is that she lives in someone else’s home. Otherwise, I guess she’d be on the street because no one else, other than our golden child brother (the one she blatantly favored and never abuse the same way she did everyone else), will put up with her head games and bullshit.
As an adult, my house is kept the complete opposite of the filth I grew up in. It is visitor ready and clean at all times. No bugs, no feces, dishes are never left in the sink overnight, laundry isn’t left in the washer, when something is broken it is quickly fixed or replaced, etc. No one would ever be grossed out at an offered cup of coffee in my house, short of not liking coffee.
Some would say this level of living in filth is a sign of some sort of mental illness. As you put it, depression or hoarding impulses perhaps. Other times, it can be a sign someone has never been taught to prioritized having some pride in their home and belongings. I’m assuming that may be the case with your friend. It just isn’t a priority for her.