I'm just going to second a few of the things sugarmom posted, because I've myself dealt with a diabetic kitty.
No no no on the "any senior food" thing. The vet is WRONG, and I cannot emphasize that more. Senior foods tend to be loaded with carbs, and this is BAD.
The pate formulas by fancy feast are actually pretty good for diabetic cats as far as storebought food goes. Many of the people on the forum dedicated to diabetic cats I used to read used them. You want to stay away from the formulas that are chopped up and with gravy, and the ones that that contain fish. I will post the website later in my post and they have a list of food that are acceptable.
Calories are also important for diabetic cats. Do NOT overfeed because an overweight cat will have more issues processing insulin, and you also do not want to feed a whole ton at once as it can cause a spike in blood sugar. Many people with diabetic cats feed 3 times a day so the meals are smaller, and also so that during the day the blood sugar does not drop too low due to the insulin shot.
Do please find out what kind of insulin has been recommended by the vet. Many vets mistakenly prescribe caninsulin, which is actually for dogs, or even humalin, which is for treatment of people, because they believe there isn't a difference or is not anything else available. Caninsulin is actually indicated for treatment of cats by the manufacturer, but it is NOT the best treatment.
I was going to recommend PZI, but I see from an article that it's being discontinued
. That is quite unfortunate since many people had great success with it.
There's an article here on insulin http://www.felinediabetes.com/starttreatment.htm
The vet put him on insulin so mom will be giving him shots twice daily. She (the vet) also recommended that she (mom) change his feeding to twice a day (she's (mom again) in the habit of leaving food for him 24 hours a day).
Please find out how much and what type of insulin the vet has recommended. And did the vet recommend testing the blood sugar before dosing? This MUST be done. Blind dosing insulin is DANGEROUS, and I say this from personal experience. Our former vet recommended to my mother in law (who had care of the cat at the time) to give a certain dosage of insulin twice daily. He did not even mention that you CAN test a cats blood sugar with a simple human blood sugar testing device. Due to this the cat had dangerous drops in blood sugar, several times, resulting in him passing out and having to be taken to the vet for treatment. I do believe this mistreatment of him after being diagnosed lead to his premature death, because if he'd been given proper treatment from the start his body wouldn't have been put under nearly so much stress.
This website, http://www.felinediabetes.com/
, is a great source of information and has a message board with a licensed veterinarian that has extensive experience with feline diabetes. I would suggest your friend go to it and do plenty of reading if she wants her kitties treatment to go smoothly.
A cat with diabetes can live a long time with the right treatment. My aunt had a siamese who lived until 19 years old taking pills for his diabetes most of his life.