May we know what are the figures of your friend for the location of the CG?

Does he have any values for the incidence angles?

The book that I referred to in post #7 explains that, for tandem wings, the lift effect of the aft wing can be considered as 80% of what it can be.
That is due to the shade effect of the forward wing.

Similar thing happens in the canard layouts, with the difference that the forward stab does not shade the full span of the aft main wing.
Note that the values shown in the on-line calculator for canards have a reduced area value for the main wing in order to compensate for the shading effect.