Is the questioner considered sinful for giving her eyebrow tweezers to her colleague, knowing the ruling on plucking and having warned her colleague, and what should be done to seek Allah's pleasure?

Plucking (al-Namṣ) is forbidden, and it is not permissible for a Muslim woman to undertake it except for a legitimate Shar'i reason. Whoever does so must repent to Allah. If you give her this tool knowing that she will use it for a forbidden act, then you have assisted her in wrongdoing, and you must repent, as Allah Almighty says: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression."