What resource should we use to direct confirmation candidates who are cohabitating?

I recommend referring this couple to the Catechism of the Catholic Church, which states, “To receive confirmation one must be in a state of grace” (CCC 1310). Cohabitating couples (understood to mean unmarried but living together as husband and wife) are not properly disposed for the sacrament of confirmation.

Preparation for confirmation should aim at leading the Christian toward a more intimate union with Christ and a more lively familiarity with the Holy Spirit—his actions, his gifts, and his biddings—in order to be more capable of assuming the apostolic responsibilities of Christian life. To this end catechesis for confirmation should strive to awaken a sense of belonging to the Church of Jesus Christ, the universal Church as well as the parish community. The latter bears special responsibility for the preparation of confirmands. (CCC 1309)

Though the state of grace is not necessary for the validity of confirmation, it is necessary for confirmation to be received lawfully.