From L. Gregory Jones and Kevin R. Armstrong, Resurrecting Excellence: Shaping Faithful Christian Ministry (2006), 65:
Why do we call them "holy" friendships? We do so for two reasons. First, these relationships are often unlikely to be developed apart from a mutual attraction to the gospel life. They bring people together from different backgrounds and histories, with diverse hopes and fears. And, second, they are oriented toward discernment and deepening of Christian vocation, as well as nurturing growth in the Christian life, toward our learning how to live as holy people. It is not that these friends must already be holy; indeed, given the shape of Christian life, that would be impossible. Holy friends are our companions on the journey of learning to desire and love God truly and faithfully.
How do holy friends shape us in our discernment, and in our growth? Holy friends are those who, over time, get to know us well enough that they can challenge sins we have come to love, affirm gifts we are afraid to claim, and dream dreams about how we can bear witness to God's kingdom that we otherwise would not have dreamed.