My 94 year old mom lives with my husband and I. She has chronic diarrhea. I have to cut off her depends and help her get cleaned up.

Yes, so disgusting. I could NOT do it! Found diarrhea clothing in my mom's hamper and started crying. Hubby saved the day and did the laundry. I don't know what I would have done without him. Oh yes I would have thrown it all away. You are a saint.

But to your issue. How long has this been going on? I agree with psyllium husk. Supposed to help either way - too loose or too hard. Great fiber source.

Meds - new ones? Ones known to have diarrhea as a common side effect? Maybe some can be reduced, eliminated and swapped out with something else. Have you talked to her doc about this yet?

Foods - my mom would get diarrhea if she had ice cream (or other unfermented dairy). This was a relatively new development in last 5-10 years, never an issue when younger. I would cut out foods that are common allergens, like dairy and see if that helps.

Try the BRAT diet (look it up).

If nothing helps, I would hire help or place her somewhere. This would be a total showstopper for me.

Is she on metformin by any chance?

Here is a legitimate website that discusses the use of psyllium husk. I would advise you to ask your mom's doctor about it before giving it to her, as TopsailJanet suggested.
https://www.webmd.com/drugs/2/drug-56088/psyllium-husk-fibre-oral/details

I agree with all 4 previous answers. Determining the cause of the diarrhea would help, if you can. It could be diet or medications for any diseases she has. My wife has frequent bouts of mushy stools and less often, diarrhea. I give her Loperamide which works well for about 4 days, but it sometimes leads to constipation that eventually resolves. 
I cut off her diaper and roll her over (one leg at a time) to remove it. I also use a cloth, washable, overbed covering: Amazon-Incontinence Bed Pads - Reusable Waterproof Underpad Chair, Sofa and Mattress Protectors - Highly Absorbent, Machine Washable - for Children, Pets and Seniors (18x24 (Pack of 4), Blue) and on top of that, a disposable underpad (29"X35") to protect the mattress cover. I can usually contain the mess by folding the underpad, while lifting her legs one at a time to remove it. I still sometimes have to wash the overbed covering if there is spillage onto it. Cleaning her with baby wipes and toilet paper is the most difficult and delicate job.

She could have c-diff infection. It turns your bowels to water.

Got her to the doctor.

Cut out coffee altogether. Always wear protective gloves when dealing with body fluids. That includes feces.

I hope something helps, but she may have reached the point where she needs professional care.

Does she drink coffee, cut it out. Apple juice too. Bananas are good for binding as is cheese.

If this is something new or has gotten worse, talk to her doctor. This can dehydrate Mom and cause other problems.

My mom was having a lot of diarrhea. Her Gastro PA suggested I increase her fiber capsules (psyllium husk) from one to two a day. It worked. I can't say it would worrk for you. Mom has always had a lot of GI problems, so has a regular appointment with them. You should probably consult your mom's GP or get a referral to GI specialist.