(emphasis added)
The assumption that knowing equals doing, is a classic sign of not having experienced what she was talking about. 
Being too ill to know, and too ill to do, are separate things.
I went to college with a woman who stank something awful. She smelled like, and I am not making this up, formaldehyde. It lingered in her room, until you could even smell it outside her room. Her roommate was friends with my roommate, and so she used to stay in our room, in a sleeping bag on a mat on the floor, until we finally convinced the RA there was a real problem, and she got moved.
At first, we thought it was because she was from another country, and didn’t shower very often, but I’ve known people from other countries who didn’t shower very often, and they smelled like ass, which went with them when they left the room; they didn’t produce an odor like formaldehyde, with the sticking power of Napalm. I think in retrospect she must have had some kind of metabolic problem, and I feel kind of bad for making fun of her behind her back. Not that I think it could have hurt her to have showered every day, but maybe she still wouldn’t have smelled so great anyway.
If anyone knows of a metabolic disorder that makes you smell like formaldehyde, I’d love to know what it is. She was Deaf (as was everyone but me-- this was at Gallaudet), so that could be involved to. There’s at least one Deafness syndrome that involves kidney disease, although I don’t know that it involves a body odor, much less one that smells like formaldehyde.
I do know that there are metabolic disorders that produce odors. One produces an odor that supposedly smells like rotting fish. I’ve never met someone with this. It usually occurs in women, and usually starts around puberty. It can be controlled to an extent with diet. There’s another that is actually called “Maple sugar urine disease.” People who have it smell like maple syrup. When they leave a room, people who come in ask “Why do I smell pancakes?” I guess there’s worse BO, but given the choice, one would rather have no BO at all.
I had a very good friend who used to get a funk toward the afternoon. She just needed to shower twice a day. But it was pretty unpleasant. Sometimes she’d ask me if she smelled OK, and say “Be honest.” I don’t know what caused it. She was a vegetarian, and said it used to be a lot worse before she stopped eating meat. I believe it. My husband was completely without odor when he was a vegan. He never wore deodorant, and could skip a shower once in a while if he was running late, and no one knew. When he started eating eggs and dairy after Army Basic Training, he started getting BO after he ate garlic. Only then, but wow, is it bad.
So yeah, it’s possible this woman has something going on. She may respond to “You stink” with “I have X condition.”
It’s not a matter of forgetting, but of that specific task being one which is - not doable.
When my father (unemployed and preparing a civil service exam) and my brother (bipolar) were depressed, they knew they had to study. Knowing they did have to study didn’t make it any easier to concentrate.
I chose to be brief and vague with my explanation because it’s a long and complicated issue, as mental illness tends to be. I doubted I could get away with not mentioning the reason for my odor problem.
As DavidwithanR said, knowing is not the same thing as doing. But yes, I am physically disabled as well. Showering is not the simple and easy task that it is for other people.
Aside from my depression, I also suffer from severe anxiety (both social anxiety and general anxiety). I strongly dislike leaving the house/being out in public, because it’s physically and psychologically taxing. Generally I only go out when it’s necessary, like doctor appointments or grocery shopping. Most often I shower when I know I have an appointment the next morning. At all other times, I fall into a vicious cycle: “I can’t be in public today, I smell bad, people will notice,” but then “I don’t need to shower, I won’t be going out so it doesn’t matter to anyone.” I have to force myself to shower (depression = no motivation to take care of myself), but it’s hard to convince myself it’s necessary when I don’t even want to be around other people in the first place.
In case you’re wondering, yes, I do live with someone else — my mother. But she isn’t a frequent showerer either. I can’t really speak as to her reasons, but probably she’s perpetually too tired and/or too sore from work. She’s got her own problems both physical and mental. Let’s just say I know where I got it from.