Cat will only eat Whitefish canned food

I started caring for my brother's 13 year old cat, and the only thing she wil eat is Friskies Whitefish canned food it seems. I've even tried the Tuna, and she doesn't seem to go near it. He said he always used Friskies, but should I try another brand? Is it good for her to eat the same thing all of the time?

I even tried the Friskies crunchy treats, and she wouldn't touch them.

I've heard that tuna's not too good for cats, yet it's the main ingredient in most cat foods. Fish in general shouldn't be fed too much as it can cause some problems, eyes & I don't know what else. How long has she been eating this food? & how's her health?

She did try the Tuna after I left it out for about an hour, and then she licked her lips afterwards.

But she doesn't seem to like the Chicken or Liver kind, which is another reason I was wondering if I should try another brand.

From what I understand she has been eating nothing but Friskies most of her life, she might be just used to it. But I really don't want to give her only fish, espeically just one kind. She's a sweet cat, but seems to be a really picky eater.

As with all changes in diet, it should be done gradually. Try mixing a new food in with what she likes, maybe at the rate of one tablespoon at a time. Over the next week or so add a little more of the meat variety & she may become used to it.