Cyt....sorry your friend is having this problem....it's really a very common one.
I believe that they often simply forget how to get cleaned up. The sequence...take off dirty clothes...turn on shower...etc........it's overwelming to them. My MIL (who was the tidiest woman i ever knew!) just absolutely quit making any attempt to get herself cleaned up. When I realized she'd gone several weeks in the same dingy nightgown, and no shower, I felt awful! Her 3 sons made no attempt to help her, and I just wasn't aware. I started going over a few times a week to clean her up...at first she hated it...but I just kept talking softly to her while undressing her and helping her into the tub.
Those caretakers are usually right on top of things...it's unusual where my own Mother is for her to be unkempt when I visit. She had been on Hospice for 2 years...(just taken OFF because she gained 1/2 pound...sigh)...and I wonder if some of the reason she no longer is in their care is because I complained one week when she hadn't been showered in several days. According to the regular caretakers (who love Mom) the responsibility of 3 showers a week was on Hospice, not them. I returned to the facility a few hours after discussing this with a caretaker, and Mom had been showered.
I remember I had to make sure buttons were already buttoned on Moms blouses after laundry...just so many obstacles for them that we don't even realize. Those old smelly jeans were familiar to your friends MIL...he has to make sure to remove soiled stuff when he leaves. She really doesn't hate him.........again, a familiar person in her life that she can take her frustrations out on.
Often, I cried all the way home for the first years of Mom's placement. It's in the 10th year now.........so many things we've both endured! Hope this helps!.....Pam