If Geico accepts his estimate without having their appraiser get an agreed price it's their fault and he has every right to seek a better deal and he can keep the extra. Geico knows this and probably has a policy of accepting estimates on a certain level of losses.
And as far as Keogh's below, the value of your vehicle is not reduced with a minor loss like this. I don't know where some of this stuff comes from. It's made up I suspect or people think they are experts because they had an accident once and dealt with an adjuster.