Your customer is correct. Components should not fall a board during a reasonable drop test.
A shear test poorly represents a drop test. A drop test represents a drop test very well.
If you want to proceed with shear testing, search the fine SMTnet Archives. There certainly has been discussion on this type of testing previously. The "typical numbers" for shear testing a particular component that anyone gives you are meaningless. The numbers are driven by the rate shear, setup of the test, and configuration of the component being sheared; not the material or size of the solder deposit as you would expect.
Consider sending these boards to a failure analysis laborator to better understand why your solder is not holding well.
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