It sounds like a rather common issue, either from the diabetes itself or from the meds. She and her doc should work together to find her a medication that doesn't have that effect, or to get her a prescription for something that can ease the runs. She shouldn't have to be continually dealing with accidents, that's for sure, but she really should be taking her meds.
"Over time, high blood sugar can damage the tiny blood vessels and nerves in your body, including your digestive system. A speed-up or slow-down of the process in your intestines could result in diarrhea or constipation. Diabetes medications, certain foods, and related illnesses can cause diarrhea, too.
"When diabetes damages the nerves going to your stomach and intestines, they may not be able to move food through normally. This causes constipation, but you can also get alternating bouts of constipation and diarrhea, especially at night.
"Fluid that lingers in your small intestine too long can allow too much bacteria to grow. This could lead to bloating, belly pain, and diarrhea.
"Nerve damage in your large intestine may let fluids move through too fast, or cause problems with absorbing and releasing fluid. If that's the case, your poop could be more watery, and you'll need to go more often and urgently.
"Metformin is in medicines many people take for type 2 diabetes. It helps lower your blood glucose and makes your body more sensitive to insulin, but it can also cause nausea and diarrhea when you first start taking it or raise the dose. Those side effects usually go away in a few weeks.
"Diarrhea is a possible side effect of other diabetes drugs, too."
My thought is, though the article mentions diarrhea a lot, it doesn't say "accidents" are an inevitable side effect. This makes me think that it's possible to find a drug (or a combination of a drug and something to calm the bowel) that stops this side effect of her condition.