Answer
The testimony is going to hurt their case.
Work Step by Step
We multiply the density of the beer (which is equal to that of water) by the increase in height and by the cross-sectional area of the keg to obtain:
$ \Delta m = 1000 \times .012 \times \pi \times (.2^2)=1.51kg\approx 53 \ oz$
This is more than the amount that the person would need to be above the legal limit, so this is going to hurt their case.
