My 91 year old friend is always itching; any suggestions as to why?

My mom had a similar rash, exacerbated by heat. Sometimes it would look a little better but then if she was sweating or overheated, it would flare. Nothing helped and everything the drs and nurses tried seemed to aggravate it. Took her to a dermatologist and turned out to be a fungal rash and required prescription treatment. I believe they did a course of oral diflucan and she never had it happen again. I’m not trying to diagnose your friend, just throwing out another possibility for your consideration.

Itching (pruritus) is one symptom of chronic liver disease, though not everyone with liver disease develops it. It can be a localized itch, such as your friends or it might be an all-over itch. Either way, it can lead to a distracting, often overwhelming, desire to scratch.
A little itch now and then is no cause for concern. But continual itching can interfere with sleep and lead to a host of other problems. When that happens, it becomes a serious health concern.
It’s important to avoid scratching that itch because it can make matters much worse. Keep her fingernails short so that if she does scratch, she is less likely to break the skin and open the door to infection.
If she tends to scratch a lot during the night, she can try wearing gloves to bed - as long as doesn’t remove them! .
Here are some other things she can do to prevent skin irritation and ease itching:
Use warm or cool water rather than hot water for showers and baths.
Try not to spend too much time in hot environments or in the sun.
Choose mild soaps that don’t contain added fragrances.
Use gentle, fragrance-free moisturizers to combat dryness.
Apply a cold, wet cloth to the itchy area until the urge to scratch eases up.
Avoid substances or materials that irritates her skin.
Wear loose-fitting, breathable clothing.
Use a humidifier during the dry winter months.
I’ve found white vinegar can help reduce some itching too.
There are anti itch topicals and oral medication - though if she has liver disease oral treatment would need monitoring.
Antihistamines whilst good for other itch related issues - don’t usually help if caused by liver disease - however they can aid sleep despite itching.
https://www.webmd.com/skin-problems-and-treatments/guide/why-so-itchy
Might be some advice there too
Do hope you can get some help for your friend - good luck!

Are u in the South? When I visit relatives in the South, I find they use water softeners. It takes me a while to get the soap rinsed off my skin and hair. If this may be a problem, then maybe she isn't rinsing well enough.

Another thing I thought is the chair or sofa she sits on. The material maybe the problem.

Or it could be something as simple as loose hair that falls in the back of one's shirt on the inside and the same with falling into slacks while dressing or using the bathroom..

I have long hair, and just one broken piece of hair can drive me crazy until I can find the culprit.

Geriatric pruritis is what it's known as. And the cure is like looking for a needle in a haystack, literally. The only thing that worked, eventually, for my mother, after spending a lot of money on a lot of Rxs, ideas, lotions, potions and antihistamines, was Dermoplast spray. Which is a spray you can buy over the counter which numbs the itchy area temporarily. Mother's itching went on for about a year until one day it just magically stopped. Now it's back in her scalp, where no Dermoplast spray can be used. Of course. Because my mother is the queen of finding ailments that nothing can ever really fix or cure. :(

The moral of the story is: It's always something for a person with advanced old age.

Use vinegar in the rinse cycle and this will ensure that there is no soap residue in the fabrics.

My husband gets itchy during the winter and we use a 50/50 mix of sweet almond oil and jojoba oil. No more itching.

That would be my 1st step after verifying that this didn't start after a new medication was introduced.

Best of luck, that is torturous.

I agree with changing the laundry detergent. I recently had to do that for my husband. Also, liver disease can cause intense itching.

I go with her detergent since its her back and bottom. Has she changed it? Maybe its the soap she uses. Deoderant soaps tend to dry the skin out. Ivory, believe it or not, dries the skin out. Dove is good, but DH complained about itching from it. We now use Oil of Olay and neither of us has complained about itching.

Does she have gas heat. It is a dry heat and like said, u need a humidifier to put moisture in the air.

Another thing I suggest is rinsing clothes 2x. My washer doesn't have a second rince cycle but some do. This will help to eliminate more of the soap. Is she using too much soap?

All the suggestions are very good.

Grandma1954, good suggestions, especially the issue of hydration, and even more so since it's winter.  

That reminds me of another possibility:  dry air.   Sandieg, do you have a hygrometer?  If not you can get one, fairly reasonably. It measures the amount of moisture in the air.  

The furnace probably has a humidifier on it as well.  Check the settings.   You can also buy a room humidifier for her bedroom.

An air mattress might help, there is a continuous flow of air that help cool the skin but since there is also movement it can help prevent pressure sores.
A good moisturizer might help as well.
A talk with her doctor might be in order. Some medications can cause dry skin or there may be another medical problem that is undiagnosed.
Grooming her nails should also be important. Less likely to break the skin with shorter rounded nails and making sure under the nails is/are clean so that if the skin is broken there is less likely to start an infection.
And keeping her hydrated will also help keep the skin in better condition.

Could be any kind of laundry detergent, dryer sheets (toxic) if they're used, soap, skin cream, preservatives and other ingredients in creams, allergy to meds, and probably others I haven't thought of.

Start changing one element first to see if the itching is solved.  Has she been on new or a different med?  How does she keep clean?   Washing with soap, no rinse soap?   Are any lotions such as moisturizing lotions used?  

The fact that the itching apparently is only on her back and bottom makes me wonder as well if she sits all day, or if some of the clothing she wears is polyester or potentially abrasive (like some synthetic clothing can be), or if she sits in a wheelchair, or something plastic.   

Synthetic clothing (such as 100% poly) can prevent skin from breathing properly.  I used to use it in clothing I made and learned the hard way. 

Are her back and bottom warm only after sleeping?   If so, I'd concentrate first on washing her nightgown or PJ's, in a detergent that doesn't contain fragrances.      And don't use dryer sheets.    You might also put a towel or something not synthetic on the back of the chair in which she sits.

If you're interested in learning more about toxins in skin care and dryer sheets, the Environmental Working Group has very good information.