There is really no such thing as a "good Muslim" and a "bad Muslim", these are labels we have invented. A person is a Muslim if they profess the shahadatayn. However, a Muslim will be judged in the next life like anyone else and a person who has harmful behaviour or bad ethics will be held responsible for that.
There are a number of hadith saying not to look at someone's outward signs of religiosity (such as praying or fasting) to judge their character but rather to look at how they act with other people. This is because acts of worship can become habitual, be feigned, done with a dead heart, done out of custom or to please one's family or community, etc.
However, acts of worship are there for a reason, and can elevate a person; whereas leaving them aside can make them more vulnerable to wrongdoing. Perhaps, inshallah, her prayers will help her become a better person.