Can a type 1 become a type 3c?

Your friend's pancreas wouldn't have alpha or beta cells. There would have to be some illness (other than diabetes) or damage to panc either by operations or accidental damage to be type3c

interested to know what other illnesses a person with type 3 would have to have ....

According to this paper type 3c diabetes: is normallyy defined" by the absence of autoantibodies (so not type 1), and both exocrine pancreatic insufficiency and typical morphologic pathology" ( I take that to mean typical changes to normal pancreas so 'damage')

Your pancreas has two important functions: endocrine and exocrine

As diabetics we think most about the islet cells which contain beta cells which release insulin into the blood stream but they also contain alpha cells that secrete glucagon . There are alsp delta cells which may act as a signalling system . This is what is called the endocrine pancreas.(

However your pancreas is also what's called an exocrine gland. It plays a very important part in the digestion of food. There is a large duct(or tube) down the middle of the pancreas leading to other ducts and eventually into the small intestine. The exocrine cells in the pancreas produce pancreatic 'juice' containing enzymes necessary to break up fats, proteins and carbohydrates. It also releases bicarbonate which neutralises the acidic acids from the stomach. These products are passed into the duct system. If these ducts are blocked, or missing then many foods can't be digested properly.

This short video is quite a good animation ( a tiny bit of chemisty in the middle but not too much)
The functions of the pancreas can be damaged by some conditions and sometimes pancreatic surgery is necessary

The paper linked to above said that that exocrine pancreas damage is caused by (in order) ' chronic pancreatitis,(76%), then pancreatic cancer (9%) ,hemochromatosis (8%), cystic fibrosis(4%) and finally previous pancreatic surgery(3%)'
Chronic pancreatitis has itself various causes. http://www.mayoclinic.org/diseases-conditions/pancreatitis/basics/causes/con-20028421

if you had your whole pancreas removed, not only would you not have the islet cells but you wouldn't have the enzyme producing cells and ducts. Otherwise pancreatic surgery or some other condition that causes damage will have varying effects (you could still have some function,just not working as well as before)

Since, there can be varying amounts of damage ,some people with T3c need insulin, some don't, some need to take pancreatic enzymes with meals, others don't. How difficult it is to control must be variable.
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T1 is normally considered to be caused by an autoimmune attack on the beta cells but can also seem variable. Sometimes after time the alpha cells releasing glucagon and the signalling system between them doesn't seem to work as well as it should. Also people can develop nerve damage and this can sometimes effect the way their stomach and intestines work. If these don't empty predictably, it can lead to difficulty in control.