Is this measurement of a cricuit's output impedance valid?
Consider the inverting op-amp circuit as driving to bring the inverting-input to the same potential as the non-inverting input. i.e. V- = 0 V. 
To do this I2 = I3 (the feedback current = input current). 
Since R2 and R3 are both 1 kΩ then the voltages across them are identical. Therefore, since Vin = 1 V then Vout = -1 V.
There is no error on the output voltage due to the 70 Ω output impedance of the op-amp provided the op-amp is running within specifications.


I think your advisor is incorrect.