Didn't find any mention that U-Turns were legal or
illegal. However, I did notice § 545.062 which would seem to indicate that the other driver was at fault.
§ 545.062
FOLLOWING DISTANCE. (a) An operator shall, if
following another vehicle, maintain an assured clear distance
between the two vehicles so that, considering the speed of the
vehicles, traffic, and the conditions of the highway, the operator
can safely stop without colliding with the preceding vehicle or
veering into another vehicle, object, or person on or near the
highway.
EDIT
With that said, based on the diagram, I would have to say that in California, the U-Turn would not have been legal. I understand OP is not IN California
