As far as I know what he did is legal as the truck he was passing had his signal on to turn right. The part that makes it completely his fault is he should have seen that other truck and stopped before he passed.
I agree, the rider is not legally at fault. Because of the differences between Africa and Canada, this situation translates to a rider passing a left turning vehicle on the right, which is completely legal as long as there is enough room for both vehicles to fit on the paved roadway. It might not have been smart, but to me it was legal (when comparing the situation to the HTA).