Hunger is a natural thing. It doesn't just vanish after surgery. Some people experience different levels of hunger after surgery, but in the grand scheme of things, it won't be as significant. The thing to remember is for the first few months you will probably be eating less than 800 calories a day, and that is next to nothing. The fact that eat that few calories and not be absolutely ravenous and crazy is one of the amazing things about the surgery. You still get hungry because your body wants to eat, but compared to how you would feel trying to restrict yourself to 800 calories before surgery, you usually can't even compare it.
The surgery still requires a commitment to changing your relationship with food and making healthy choices for live. That doesn't mean you have to exclude all foods all the time, but a great majority of the time, to sustain this long term, we've got to eat differently and move differenting, and think a bit differently. It's not always easy, but the surgery provides an incredible avenue and opportunity to make huge changes in your life and drastically improve your overall health and quality of life. Don't lose site of that. It is something that people who try to diet almost never achieve.
The surgery isn't successful because you have a smaller stomach and eat less. It is successful because the side effects of the surgery cause changes to how our biology reacts to certain hormonal signals in the brain and other tissues throughout the body. This change in signal reception allows your body to lose fat without your brain trying to fight against like it did prior to surgery. Our bodies are not designed for weight loss, they are designed to protect it, and when we are obese, the negative effects are compounded.
Regardless of what you eat, your body will lose some weight after surgery because your body wants to get to a lower weight. It will actually burn more fat than normal to get there in many cases, but at some point, that will turn and bad food choices can send a person back to the beginning, but don't be too judgemental of your friend. She may not be eating those foods all the time. I have pizza every now and then, albeit not the same sort of pizza I had my by previous life. Still, her experience and choices will be very different than yours.
If you are interested, here are some posts on reddit that dive a bit deeper into why diets fail, why the surgery works, some info about stalls and why WLS sometimes fails:
Why Diets Don't Work and Why WLS Does & More …