Hello and Welcome to Healthcaremagic,
It is good to see you are concerned about your family member and are seeking out understanding in an attempt to ease her suffering.

I can give you some concrete ideas on how I would treat her if she were my patient and this is all the information I had about her. First, it sounds as if she may be having malabsorption problems with foods in her diet, particularly ones that are high in fat. The diarrhea causes her food to pass through the intestinal track so quickly that she also is not getting the vitamins and mineral absorption needed and her electrolytes may be off as well.

Several issues need to be addressed symptomatically. First, fluids. She is losing a great deal of fluid with the diarrhea so suggest she drink 10 glasses of fluid per day. Including an electrolyte drink once per day may help her feel better. An electrolyte drink as simple as no added sugar Gatorade or such is fine.

Before I proceed, I want to tell you that this is a common problem for people that have their gallbladder removed-diarrhea that may last for years.
One theory is that there is excess bile after the surgery which acts as a laxative, so avoiding prunes and taking anti-diarrheal medications may help. Also avoid lactose (look for lactose free diet plan).
Specific Instructions: 1)Avoid fatty, greasy foods
2)Avoid caffeine
3)Avoid sugar
4)Avoid lactose, but assure adequate calcium intake. Taking Lactaid, adding lactaid pills to milk to break down lactose, consuming Greek yogurt low in lactose all may help add calcium while getting rid of bothersome lactose so dairy may be tolerated without promoting diarrhea.
5)slowly add fiber to diet up to 25 grams. If fiber is added to quickly bloating and gas will occur. Fiber can be added through food (oatmeal, oat bread, well cooked or fresh vegetables, skins of fruits,etc)
6)During acute phases of diarrhea after surgery, the BRAT diet may be followed or Bananas, Rice, Applesauce, Tea/Toast, Yogurt (Greek only)
7)Avoid spicy foods, hot peppers and such
These are the dietary suggestions.
8)Dietitians, myself included, rely heavily on food diaries to pick up sensitivities aggravating bowel problems. Consider keeping a food/symptom diary and consulting a Registered Dietitian if problems persist.

Medical Care for the Miseries of Diarrhea:

Don’t wipe after a bowel movement, pat gently. This will clean without being abrasive. Baby wipes are a great way to gently clean without causing more irritation.

Soak bottom in hot tub to sooth irritation.

Consider applying healing a thick protective ointment to the sore area. It will provide a barrier between your sensitive skin and the bile/gastric acids that are causing the irritation.  You can also use ointments that are designed for diaper rash, as they tend to be thick and long lasting.

Consider medication to speed the process along, Cholestyramine--a bile acid sequestrant-- is often prescribed for diarrhea after gallbladder surgery.

I have given you a lot to think about and many helpful suggestions. My heart goes out to your cousin and I certainly hope some of my information is helpful.

You will need to review a "high fiber diet plan" and a "low fat diet plan" to put these suggestions in place. Hopefully, these diets can be googled or you can write me for these general plans at https://www.bit.ly/askkathyshattler
here at HealthCareMagic.

Regards to you and your cousin, Kathryn J. Shattler, MS, RDN (Clinical Registered Dietitian Nutritionist, USA)