"Now he sets down the true sign of friendship on his own part, which is that all that I have heard from my Father I have made known to you. For the true sign of friendship is that a friend reveals the secrets of his heart to his friend. Since friends have one mind and one heart, it does not seem that what one friend reveals to another is placed outside of his own heart [(non videtur amicus extra cor suum ponere quod amico revelat)]: 'Argue your case with your neighbor [and do not reveal a secret to a stranger (causam tuam tracta cum amico tuo et secretum extraneo non reveles)]' (Pr 25:9). Now God reveals his secrets to us by letting us share in his wisdom: 'In every generation she [Wisdom] passes into holy souls and makes them friends of God and prophets' (Wis 7:27)."
St. Thomas Aquinas, Commentary on the Gospel of John 15:15 =no. 2016 =cap. 15 lectio 3 (trans. Fabian Larcher, O.P., and James A Weisheipl, O.P., with introduction and notes by Daniel Keating & Matthew Levering (Washington, DC: The Catholic University of America Press, 2010), vol. 3 (Chapters 13-21), p. 111, interpolations and underscoring mine).
I was put onto this by Serge-Thomas Bonino, "Le rôle des apôtres dans la communication de la révélation, selon la Lectura super Ioannem de saint Thomas d'Aquin," Bulletin de littérature ecclésiastique 103, no. 4 (octobre-décembre 2002): 327-328 (317-350).