Is the questioner sinful if her friends go out with her while not wearing the legal hijab, given her prior knowledge of this and her attempt to advise them?

A committed woman going out with a non-hijabi woman is not blameworthy, provided that the Islamic guidelines are observed. The questioner does not incur sin if she was the reason for her going out of the house, because if the assistance is not intended or direct, its perpetrator is not sinful. It is preferable for a Muslim woman to associate with righteous women, for a companion influences their companion. The example of a good companion is like the bearer of musk, and the example of a bad companion is like the blower of bellows.