so, I had a few ideas of what different mental illnesses could be related to lack of hygiene & being a total slob... but I also happened to have a little free time on my hands, so I researched (i love stuff like that)! ADD, depression, different anxiety disorders, certain phobias, & schizophrenia are the main ones I found. depression was already mentioned in what your friend wrote, so that seems to be the winner here! depressed ppl no longer have the willpower nor the desire to look presentable.

alcohol or drug abuse could also be a part of the problem. your friend didn't mention anything about either, but I'm just throwing that one out there in case...

being socially inept is another reason you could consider. ppl with a low level of social intelligence live in their own world & can't relate well with others or have no care about how others perceive them.

idk if any of these will help you, but I hope they do!