  We consider a phenomenological model for a coupling between the dark matter
and dark energy fluids and investigate the sensitivity of a weak lensing
measurement for constraining the size of this coupling term. Physically, the
functional form of the coupling term in our model describes the decay of dark
matter into dark energy. We present forecasts for tomographic measurements of
the weak shear bispectrum for the DUNE experiment in a Fisher-matrix formalism,
where we describe the nonlinearities in structure formation by hyper-extended
perturbation theory. Physically, CDM decay tends to increase the growth rate of
density perturbations due to higher values for the CDM density at early times,
and amplifies the lensing signal because of stronger fluctuations in the
gravitational potential. We focus on degeneracies between the dark energy
equation of state properties and the CDM decay constant relevant for structure
formation and weak lensing. A typical lower bound on the CDM decay time
~7.7/H_0 = 75.3 Gyr/h$ which could be provided by DUNE would imply that it
would not possible to produce the dark energy content of the universe by CDM
decay within the age of the Universe for a constant equation of state parameter
of w close to -1.
