Does sitting next to a friend who is wearing pants in front of people constitute aiding in a reprehensible act, and is it obligatory to part ways with her in this situation?

Legally, it is forbidden for a woman to wear trousers in front of non-mahram men, and this objectionable act must be denounced. As for sitting with someone who is committing an objectionable act while they are doing it, it is not permissible except out of necessity. However, when they are not actively committing the objectionable act, sitting with them is not forbidden in principle. Avoiding them (hajar) is conditional on there being a benefit; if there is a benefit, it is legislated.

Based on this: - It is not permissible to sit with a woman who displays her adornments (mutabarrijah) in front of men without denouncing the objectionable act, except out of a necessity that requires staying. - Sitting with her when she is not displaying her adornments is not forbidden, but it is preferable to avoid it if there is a benefit in deterring her from disobedience.

Some jurists consider sitting with evildoers to be forbidden. Al-Haytami considered sitting with drinkers of alcohol and evildoers as a form of encouraging them, and a major sin, especially if it is with the intention of following them. He also considered sitting with corrupt reciters of the Quran and jurists to be a major sin, due to the danger to oneself of inclining towards their actions. Al-Ghazali held that befriending wicked people is forbidden, as is sitting with alcohol drinkers during their drinking. While others believe that sitting with an evildoer is not forbidden if one does not intend to encourage their wickedness, but rather for kinship or a permissible need.