First, you must know there is such thing as erroneous conscience. Talk to her about it.
The conscience is (the voice of God within) and why Catholics who have a well formed conscience and a healthy understanding of what the Church is (the Bride of Christ, guided by the Holy Ghost), should be very willing to embrace the teachings of Humanae Vitae. But was Humanae Vitae a divinely revealed truth intended just for Catholics? Humanae Vitae clearly answers this question in the negative. In order to see this, relevant portions of Humanae Vitae will be examined which speak directly to this question. However, some preliminary work must be done on two very important moral concepts, conscience and the natural law, in particular, the Catholic understanding of how these concepts extend to mankind in general.
Aquinas in Disputed Question on Truth, On Conscience, tells us that while a conscience always binds, it does not necessarily excuse. There is such a thing as an “informed conscience” since one’s conscience may be badly formed and one may be culpable for that. The binding nature of conscience comes from judging that something should be done or not done. By conscience we judge that something done is well done or ill done, and in this sense conscience is said to excuse, accuse, or torment. [ST First Part Q79 A13].
Aquinas says that every kind of conscience binds whether correct or erroneous. This on the surface appears disturbing if one fails to ask, “Is the action obligated by an erroneous conscience good?” Aquinas did not miss this point. He argues that an erroneous conscience is one that does not know, i.e., ignores what is truly good, evil and indifferent. He then considers that since you can only do what you know you ought to do, it follows that an erroneous conscience excuses as well as binds, all the more so because you cannot be held responsible for an act you do not know you are committing. He quickly qualifies this statement, however, by observing that there are degrees of ignorance and not every kind of it renders an act involuntary. You can be responsible for your ignorance.