You could say you believe your faith because it is true, not because it is convenient. You might mention that as she is a relativist, she is rigid.
This is copied from EWTN from a Q&A faith forum:
The first three (or is it four?) chapters of C.S. Lewis’s Mere Christianity convinced many of my ethics students that relativism is false. There are some very good and readable arguments there, and they do work. I highly recommend it.
C.S. Lewis’s most powerful argument is this. One can only have a discussion of morality if both people are talking of the same thing. If each is talking of something else (e.g., I am talking of morality-for-me and you are talking of morality-for-you), then there is no possibility of disagreement and hence of discussion. Indeed, when we look at people having moral discussions, we find that they do agree on basic principles. For instance, one may say to another: “Give me a bit of your apple because I gave you a bit of mine yesterday.” The other is not likely to respond: “So what that you gave me a bit of yours?” Both people accept a duty of gratitude, but one may try to weasel out of it by giving some exception (e.g., “Yours wasn’t very tasty”).
Another argument I’ve found very powerful is this. If morality is just a matter of your feelings, as the relativist generally claims, then your moral feelings are always, by definition, right for you. What you feel is right is right for you on the relativist’s view. But this means that you are infallible about morality (for you). Since you are always by definition right about morality (for you), you never have any reason to change your moral views. Therefore, the consistent relativist has no reason to ever change his views–because he is always already right (for himself)–and so consistent relativism leads to rigidity and intolerance. This conclusion I think takes my students by surprise, and some of them stop being relativists when they get to it.
Moral progress and positive changes in moral beliefs require that some moral beliefs be objectively better than others. If no moral beliefs are objectively better than others, then we had no reason to abolish slavery. Slavery was right for us, and we had no reason to change it. The Nazis had no reason to stop being Nazis. All this is absurd, and so it must be that some moral beliefs are objectively better than others.
Few relativists have really looked into the consequences of their relativism. They don’t realize that if you are a relativist, you can’t criticize the Nazis, since what the Nazis did was right for the Nazis according to the relativist. They don’t realize that they themselves think some moral truths are absolute, such as the moral truth that “one should not torture little children just to hear them cry”. I think what is important to emphasize to the relativist, however, is that the moral absolutist is open to changing his mind about some moral issues. He is open to changing his mind precisely because, unlike the relativist, he thinks there is an objective truth, and so if he becomes convinced that his ideas do not match up with objective truth, he will change them. (Since the relativist’s ideas always match up with his own “subjective truth”, he never has reason to change them.) Of course, there are some things we already know with certainty (e.g., every sane person knows it is absolutely wrong to torture little children just to hear them cry, and almost everyone knows many other things, and our Catholic faith teaches us yet many more).
