there is no easy fix. even if you put the inverter after a branch circuit and rewire the other circuits to tie in after the inverter, your still crippling the circuit at 750 watts. wait till he gets a hair dryer on the circuit for a while
this unit is not even remotely designed for a whole house transfer. there is a reason why the unit is hardwired.
i think op got lucky it didn't fry. he keeps doing it it will soon enough.
i have seen these units in the field, they are design for very small loads like single computers etc.