First get her to take him to vets to rule out any medical/dental problems.

If he is healthy, your friend should put down a good quality cat food for him, if he doesn't eat it, take it up until the next meal. She should NOT offer any beef etc. He can miss a meal or two no problem.

I suspect that this kitty is just trying it on.

If the cat won't eat the food offered on that or the next day, it is worth swapping the cat food until you find one he likes.

One word of caution cats can starve to death if they hate their food, unlike a dog who eventually will eat what he is given.

So your friend has to find a suitable food without spoiling her cat with treat food.

Good Luck.