Did she get sick or did she test positive? If you have the vaccine, you can still test positive since the tests detect even inactive portions of the virus in your nasal tract and you will definitely test positive with a blood test since the blood test measures whether your immune cells can fight a sample of the virus.
Also, did she get both doses of the vaccine? If she didn’t get both doses, which are 2 weeks apart, and with an incubation time of 2 weeks, she probably was sick before the vaccine was even available. It takes about a month with the current both doses of vaccines to build up your immunity system so it is likely she was already infected. The vaccine doesn’t heal you, it prevents the effects of an infection.