Is your friend thus making the case that contraception should be permitted because the “unitive” aspect of love-making is still present? A very weak argument.
If this is, in fact, the argument, there are several points I would make.
First, when a woman is not fertile, she is not fertile because of the laws of nature (of God!), not of man. Rendering oneself infertile is not the same as being aware of one’s fertility and making use of non-fertile times. As Christopher West says, the difference is the same difference as between a miscarriage and an abortion. One is an act of God, the other an act of man.
Secondly, the argument that contraceptive love-making is still “unitive” is absolutely false. Spouses are saying, with their bodies, “I love you, but not enough to give you my fertility.” “I love you, but not enough to be open to life.” HOW can love-making under these conditions be unitive if the very nature of the love-making is self-pleasure?