Is the questioner sinful if she accompanies her friend who plucks her eyebrows, despite her warning her?

It is not permissible to accompany someone who commits a sin or to sit with them in the place where the sin is committed, unless there is a necessity. This is because a Muslim is forbidden from witnessing gatherings of sin, unless they are capable of removing the wrongdoing, as Allah Almighty says: "And He has already sent down to you in the Book that when you hear the verses of Allah disbelieved in and ridiculed, then do not sit with them until they engage in another conversation. Indeed, you would then be like them." (An-Nisa 4:140) And whoever sits in a gathering of sin and does not object to it is a partner in their sin.

Regarding accompanying someone who practices "namṣ" (plucking eyebrows), not every touching of the eyebrow is forbidden, as "namṣ" is a matter of scholarly disagreement. If your friend is imitating those who permit some forms of "namṣ", then there is no sin upon her, and she should not be objected to, because "there is no objection in matters of scholarly disagreement." However, if she is doing something that is unanimously agreed upon as forbidden, then it is not permissible to accompany her.