Let’s continue our application of the three fonts of morality to the principle of cooperation with evil. Implicit cooperation concerns the font of intention (first font). Formal cooperation concerns the font called the moral object (second font). If your act is sinful due to your intention, then change your intention. But if your act is sinful due to an evil moral object, then the act is intrinsically evil and always immoral; you must choose a different type of act, one that has a good moral object. An intrinsically evil act is, by its very nature, ordered toward an evil end in terms of morality (i.e. an evil moral object).
Formal cooperation with evil occurs when the sin of the other person is an intrinsically evil act, an act with an evil moral object. If your act cooperates with the intrinsically evil act of another person, such that your act is inherently ordered toward the end of assisting the act of the other person to attain its moral object, then your act also has an evil moral object and is therefore also intrinsically evil and always immoral.
Formal cooperation can occur by commission or by omission. As an example of formal cooperation by commission, if a group of voters take part in a propaganda campaign in favor of an intrinsically unjust law, such as a law permitting abortion or euthanasia, their acts constitute formal cooperation. (cf. Evangelium Vitae, n. 73).
As an example of formal cooperation by omission, suppose that a nurse is caring for a terminally ill patient, and she knows that the patient plans to commit suicide (which is also euthanasia in this example). The patient has obtained a pain medication that he can use to commit suicide by overdose. The nurse might cooperate formally with the suicide of that patient by deciding to omit a scheduled visit to the patient’s room, so that the overdose will not be detected until the patient is dead. This act of omission is directed at assisting the patient in attaining the moral object of an intrinsically evil act of suicide. Therefore, the nurse is committing an intrinsically evil and gravely immoral act of formal cooperation by omission.
Formal cooperation is always immoral because it is by definition an intrinsically evil act.
Pope John Paul II: “Christians, like all people of good will, are called upon under grave obligation of conscience not to cooperate formally in practices which, even if permitted by civil legislation, are contrary to God’s law. Indeed, from the moral standpoint, it is never licit to cooperate formally in evil.” (Evangelium Vitae, n. 74).
Much more on the principle of cooperation with evil and on the three fonts of morality is found in my book: The Catechism of Catholic Ethics