"They are confidantes, of course, in Racine and the Greek tragedies"

The extant Greek tragedies consist mainly of the work of three playwrights: Aeschylus, Sophocles and Euripides. They wrote during what is generally known as the Golden Age of ancient Greek theatre whose beginning dates from the period of the Persian destruction of Athens in 480BC.

Mural depicting Melpomene - The Muse of Tragedy by Simmons (1896)
Public DomainMural depicting Melpomene - The Muse of Tragedy by Simmons (1896) - Credit: Edward Emerson Simmons (1852-1931); photograph: Carol Highsmith, Wikimedia Commons

What plays, in particular, the narrator has in mind where women are each other's 'confidantes' is not clear.

Any suggestions?