Standard disclaimers apply here, but I have cat.
Tuna intended for human consumption is not a balanced diet for cats (it’s not a balanced diet for humans, either, but we’re not discusing them at the moment). In addition to the protein that is undoubtedly present in the fish, she also needs vitamins and minerals. I do not know every feline nutritional need (I doubt that anyone does), but, e.g., well-nourished cats synthesize all of the ascorbic acid (vitamin C) that they need, whilst they cannot convert carotenes to retinol (vitamin A), but must have retinol present in the diet.
Assuming that the cat has previously been adequately nourished, she can certainly survive on tuna for a few weeks. I wouldn’t go beyond a month, however. Aditionally, growing kittens need more sheer calories, and may have other dietary needs that adult cats don’t.
As a safety measure, get her off tuna this weekend, and buy a kibble that meets AAFCO guidelines. Check with the vet to see if she has special dietary needs. And do discuss your finances (or lack of same) with the vet. Too often, lack of discussion leads to the advice that should be given if the person being offered it has an income of USD10,000,000 per year (this failing is by no means confined to veterinary personnel; ask anyone what to do about anything if “money is no object”).
“Kings die, and leave their crowns to their sons. Shmuel HaKatan took all the treasures in the world, and went away.”