Allergy to dairy! If she goes dairy free for 1 week and then get her a large Dairy Queen Blizzard {heath bar is the best} but have her eat it at home. If … within 1 hour she has pain and runs to the toilet - that's a dairy allergy - I would know! I paid the price for making that mistake tonight!
Have your mother drink orange juice and Gatorade to replenish her potassium and electrolytes. In the meantime she needs to see her doctor for evaluation. Loss of electrolytes and potassium can cause nausea, fainting, confusion, vomiting, and heart problems/failure. Drinking these 2 things a lot will help her until she sees the doctor..
I think her blood pressure is dropping when she goes and so she passes out. She needs to visit her family doc and have him do a workup on her, paying close attention to her blood pressure, draw blood and check for anemia, and on top of it she's probably dehydrated from the diarrhea. He will give her meds for the diarrhea, he might refer her to a cardiologist, might even order a CAT scan to see if she's bleeding internally anywhere, and could be he'll briefly check her into the hospital and run an IV to get her hydration and electrolytes back where it's supposed to be, plus have her watched for a few days until she starts going more normally.