How could a failed left wing cause a roll to the right?
The following is how I would interpret the information given in the report - this is to some extent speculation, also because the FDR plots in the report don't seem to cover that part of the incident. I'm very intersted in how other people interpret the data.

The report states:


  The airframe remained intact (minus the leading edge), and the load
  factor fluctuated around -2 g, for approximately 6 l/2 seconds. [...] airplane heading moved 20 degrees nose left, and a 15 degree right roll attitude was reached at the end of this period.


The fact that the left leading edge of the stabilizer was lost and that the heading is moving left makes me believe that the aircraft was in some kind of a slip with the right wing further forward in the wind. The negative load factor of -2 g indicates a lot of negative load on both wings - apparently more than the structure tolerated. Both wings "stalled negatively"1 at this point. Now if one wing goes missing (aerodynamically, even if not physically), it can't provide the downforce any more and the total momentum will cause the side of the aircraft where there is still a functioning wing to rotate down. 

Because the right wing was probably pointing slightly forward (even more so if the left wing produced a lot of drag while or after failing, which seems likely), a roll to the right would make the aircraft more or less tumble over its own wing, with the wing upper surface pointing increasingly towards the airflow around a 90° right roll position. The resulting drag explains a very fast roll to a ~180° position. Starting from "10 to 15 degrees right wing down"2, that's about 160°.

Short version: Because the wings had negative load and were primarily drag surfaces at this point, everything is reversed. The side of the aircraft with less drag rolled towards the incoming airflow. In a negative angle of attack attitude, that's up in the aircraft coordinate system, causing a roll to the right.



1 3.1: Findings; 7. p. 48 (56 in pdf)
2 p. 20 (28 in pdf)