What is the ruling on a girl not advising her female friends who are immodestly dressed (mutabarrijat), for fear that they might dislike her, and is she sinful for that?

Supplicate for your friends' guidance, and treat them with affection and respect to win their hearts and influence them, because da'wah (inviting to Islam) through example is one of the most important methods of inviting to Allah. You must denounce their actions and clarify the truth to them, choosing the appropriate time and method. Do not let fear of people prevent you from speaking the truth, and do not pay attention to the whispers of Satan. Cooperate with righteous sisters in inviting to Allah, enjoining good, and forbidding evil.