Rotate texture, not on a face

Developers Ruby API
2

The rotation of a texture is not a property of the material. It is either defined by the UV coordinates on a face, or by the pixel data in the image.

Unfortunately the SketchUp API has no way to edit pixel data (do image manipulation). You would have to write the texture to an image file, run an external (platform-dependend) program and re-import the image. That is slow, requires disk-IO…
If it works for your use case to modify the UV coordinates (= makes changes to the component!, if the user is ok with that), then you could try this:
Iterate over all faces in the component. For each face:

# Get four non-collinear points on the face's plane. 
# If we took them from vertices they could be collinear, so we create points of a square using vector math.
vector_x = face.normal.axes.x
vector_y = face.normal.axes.y
p1 = face.vertices.first.position
p2, p3, p4 = p1 + vector_x, p1 + vector_x + vector_y, p1 + vector_y
# Get a uv helper that will give us the uv coordinates of the material on this face.
uvh = face.get_UVHelper(true, true)
uv1, uv2, uv3, uv4 = [p1, p2, p3, p4].map{ |p|
  uv = uvh.get_front_UVQ(p)
  # Normalize the uvq by dividing by the third q coordinate
  uv.x /= uv.z
  uv.y /= uv.z
  uv.z  = 1
  uv
}
# Rotate the texture by 90° by flipping u and v coordinates (uv is also a Point3d, so literally x and y)
[uv1, uv2, uv3, uv4].each{ |uv|
  uv.x, uv.y = uv.y, 1 - uv.x # You could also do: uv.transform!(transformation)
}
# Re-apply the texture with these new uvs:
pt_array = [ p1, uv1,   p2, uv2,   p3, uv3,   p4, uv4 ]
face.position_material(face.material, pt_array, true)
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