The basis is the node's orientation (or rotation). If you have a direction, you can just convert that into a rotation.
If you are on flat ground, then you could do something like this.
human.transform.basis = Basis(Vector3.UP, angle_from_above)
Where human is the node you want to rotate, and angle_from_above is a float in radians around the y axis.
You can also create a Basis from just a Vector3:
human.transform.basis = Basis(angle_vector_angle)