|Our back yard, by the river|
He mentioned his liking of existentialism.
I said, "Existentialists are big on free will."
To begin - the classic definition of existentialism is: existence precedes essence.
That is an anti-Platonic, anti-metaphysical statement. Plato believed persons had a pre-existing essence. Socrates, through a metaphysical Q&A, functioned as an epistemological midwife, assisting the rest of us in remembering who we already are.
For the existentialist (like Sartre, like Camus), we are left to choose our own identity. In this we are "radically free." Existentialism is a philosophy of radical epistemic and ethical freedom, in the face of an unknown future. This, also, is Nietzsche's freedom to choose an uber-morality.
Hence, free will (radical freedom, with no preexisting ontological constraints) as a core assumption of existentialism.