Theophilos, Michael P. "John 15.14 and the ΦΙΛ- Lexeme in Light of Numismatic Evidence: Friendship or Obedience?" New Testament Studies (2018), 64, 33-43.
Theophilos observes that literary commentary related to John 15:14 may see tension between the concept of friendship and the expected obedience. At issue is "whether φίλος is intended to highlight the emotional dimension of intimacy or a sense of obligation within the context of John 15 (Theophilos 2018, 34). Theophilos approaches the question by means of a survey of inscriptions on coins.
A common interpretive tradition, which guides the definition in BDAG, is that friendship is distinguished from servitude due to its emphasis on relational intimacy rather than obedience (Theophilos 2018, 34). Yet since the 20th century scholars have recognized that friendship regularly results on some level of compliance, and that this is particularly the case as stated in John 15:14. Friends may be called upon to obey, espeically if they are friends of God and God is calling for obedience (Theophilos 2018, 35).
To illuminate the concept of friendship, Theophilos draws on numismatic evidence. The inscriptions on coins typically are indicative of a substantial difference in wealth and power. Even kings would have friends who served them. Some of these patron-client relationships are made clear on coins. Lesser rulers would identify themselves, for instance, as a "friend to Caesar." Theophilos sees this as a way a ruler could legitimize his reign (Theophilos 2018, 37). The parties involved did not need to have a particular personal friendship, but there was always an element of obligation. Theophilos notes that Tacitus and Strabo are aware of people who are claimed as friends to the emperor because of friendship with some of his governors (Theophilos 2018, 38).
Theophilos goes on to cite coins with inscriptions indicating friendship with various leaders. He concludes not only that obligation to anotehr was included in the concept of friendship, but also that the vocabulary used on coinage is consistent with that in John 15:14. It would be a natural assumption that Jesus' friends would owe him obedience (Theophilos 2018, 43).