1cc is 1cc no matter the size of the needle's capacity right?
The gauge just refers to the thickness of the needle itself, it has nothing to do with the size of the syringe.
Her vet ran out of her cat insulin medicine so she prescribed human insulin. The vet isn't sure about the exact amount for each daily pair of shots.
I don't know if this conversion table will help you.
I don't think the Vet is being responsible about this at all. The Vet should provide you with all the information of this sort, and not leave your roommate to try to sort it out.
To stabilize the cat on a different type of insulin, the Vet should administer the new insulin and do a blood draw every two hours to determine the blood glucose curve in response to the new type of insulin--that's the only way to know what's going on inside the cat. Based on the response to the new insulin, as determined by that blood glucose curve, the correct dose of that insulin can then be determined.
If I were your roommate, I would call around to other Vets in the area and see who has the cat insulin and if they will sell it to you. The cat's regular Vet should be willing to do that as well. Shifting from one type of insulin to another can destabilize diabetes control, and it's better to keep the cat on one type of insulin if the cat has shown a good response to that type. I'd try to obtain the cat insulin from another Vet rather than create new problems.
Human insulin, in general, does not successfully replace the cat's own insulin, and diabetes is not well-controlled in most cats when human (e.g. Humulin) insulin is used for treatment. Bovine (beef) insulin is most similar in molecular structure to feline insulin, and it is thought for this reason, to be most effective for these cats.
I think your roommate should consider looking for a new Vet.