What is my duty towards my friend who steals, lies, and pretends to have what she does not possess?

It is obligatory for you to advise your friend and tell her frankly about the seriousness of lying and stealing, and to warn her against evil deeds. You can also seek advice from the school without mentioning your friend's name, because concealing her faults is obligatory. We advise you to stay away from her because "a person is on the religion of his companion," so do not associate except with a girl of good character and uprightness who will help you in obedience. The Prophet, peace and blessings be upon him, likened a righteous companion to a musk-bearer and a bad companion to a bellows-blower.