SceneKit, flip direction of SCNMaterial

Instead of scaling the node (which may break your lighting) you can flip the mapping using SCNMaterialProperty's contentsTransform property:

material.diffuse.contentsTransform = SCNMatrix4MakeScale(1,-1,1)
material.diffuse.wrapT = SCNWrapModeRepeat // or translate contentsTransform by (0,1,0)

To flip the image horizontally:

material.diffuse.contentsTransform = SCNMatrix4Translate(SCNMatrix4MakeScale(-1, 1, 1), 1, 0, 0)

to flip it vertically:

material.diffuse.contentsTransform = SCNMatrix4Translate(SCNMatrix4MakeScale(1, -1, 1), 0, 1, 0)

this worked for me, flipping the normal of the geometry by scaling the node it's attached to:

sphereNode.scale = SCNVector3Make(-1, 1, 1)