I'm sorry to say that food is not going to make a difference one way or the other when it comes to his problem. It will help with his diabetes, but not with cirrhosis of the liver which is what he has. All the symptoms you describe is most common and to be expected with cirrhosis.
Cirrhosis occurs when scar tissue replaces healthy tissue in the liver from the liver being inflamed for a very very long time, years and even decades in many cases. A person usually has no idea anything is happening until the liver failure is far advanced when physical symptoms such as your father has appears to tip them off there is a problem. Cirrhosis is also permanent liver damage that never goes away and never gets better. There is no cure for it other than a liver transplant.
There are reasons your father can have cirrhosis even though he has taken great care of himself all his life and never drank, used drugs, or eat a lot of fatty foods. There are many other reasons for getting it that the medical field just does not really understand why it happens to some people. I had one of those which was primary biliary cirrhosis which means that my autoimmune system was attacking the small bile ducts in my liver that eventually caused this scar tissue. I had to get a life saving liver transplant or I would have died. Your father is not wrongly diagnosed.
The internal bleeding comes from varices that forms when people have cirrhosis. These are like veins that form due to blood supply not reaching the liver like it should and this condition called portal hypertension has a lot to do with it which makes blood back up and causes these varices to grow. They can be weak, leak and even burst causing internal bleeding. There is a procedure called "banding" which can be done to greatly reduce the risk of this happening. I used to get mine checked with an endoscope about every 3 months prior to my transplant to reduce my risk of bleeding. If there is a very large one that suddenly bursts, then a person can actually bleed to death in a short period of time, so it's important to try to get rid of these as much as possible.
There is really no special diet for cirrhosis other than to avoid salt since fluid will accumulate in the abdomen as the disease worsens and salt helps retain fluids. He should also avoid drinking all alcohol. His regular diabetic diet should be just fine for him, but he can always ask his doctor or there are tons of diabetic diets on the internet for you to give to him.
His platelets are low probably because his liver and spleen will both enlarge with cirrhosis which traps platelets in the spleen making them low. Platelets have to do with blood clotting so this is also a problem if he is bleeding because his blood will not clot well which will make any internal bleeding problem worse. At first the liver will enlarge along with the spleen with cirrhosis, but as the disease worsens, then the liver will start to go in the other direction and shrink up as it dies.
Your fathers doctor may refer him to a transplant center to get evaluated for a liver transplant if he thinks he needs one yet. Anyone with cirrhosis who has a platelet count of 50 with bleeding varices should be referred to a transplant center for evaluation in my opinion because that suggests the disease is in the more advanced stage. I wish him the best.
I'm a nurse who had primary biliary cirrhosis and received a liver transplant 7years ago.