What is the ruling of Islamic law regarding the work of a biomedical equipment maintenance engineer in preparing devices for forbidden cosmetic surgeries, and what is the optimal course of action if there is knowledge of the type of procedure, or upon being asked to sterilize cosmetic tools?

If devices are used in both permissible and forbidden operations, there is no objection to preparing and maintaining them, especially if their use in the permissible is more frequent. However, if you know that a specific operation is forbidden, it is not permissible to prepare or maintain devices for it, as this constitutes aiding in evil. It is forbidden to sell anything that can be used for both permissible and forbidden purposes if the seller knows or strongly believes that the buyer will use it for forbidden purposes. Maintaining and preparing a device for a forbidden use is subject to the same ruling as selling it to someone who will use it for a forbidden purpose; therefore, it is not permissible except out of necessity if you fear harm to yourself.