Cohabiting Couple Doesn’t View Fornication as a Sin
What can you say to a couple who loves God and others but can't grasp that living together is a mortal sin punishable by hell?
I infer you mean that they’re being sexually intimate and thus committing the sin of fornication.
Fornication is an intrinsic moral evil (CCC 2353), and yet mortal sin has three components (see CCC 1854-64). Given the law of God written on every human's heart (Rom. 2:14-16), we shouldn’t pastorally presume upon mitigating circumstances in such cases.
You say they love God. Are they Catholics or other professed Christians? If so, remind them that Scripture teaches that those who fornicate without repenting cannot inherit the kingdom of God (see 1 Cor. 6:9, Gal. 5:19).
If they don’t accept the Bible in this regard, caution them against the sin of presumption and inserting themselves as God in judging their own behavior. In addition, appeal to them from a natural-law perspective. The possibility of procreation is an indication that a couple should be committed, for it is harmful to all concerned, particularly to a child, to procreate offspring in a non-marital context.
In this regard, if needed, make a special appeal to the woman in this case, or have a good Catholic woman who is close to her. That might be what works and makes the man involved think twice about it means to be a loving man of God.