With the limited information you were able to provide I tried checking it out on
here by doing a search for 68 california special mustang. The average price seems to be around $14k-$15k for a top of the line car. Depending on the actual shape of your friends car it could be somewhat less.
With that in mind, if the car is as solid as it sounds it very well could fetch the $10k he was wanting.
That is just an educated guess on my part and some of these other classic guys might be able to help you out more.
------------------
1966 Baby blue Coupe, 289, Manual 3-speed, Edelbrock 650, Flowmaster 3-chambers, and minor upgrades. Interior work includes custom paneling, custom dash dress-up(I refuse to cut anything out of the original), Rewiring most of the electronics, Porsche 911 seats installed soon, and Stroker kit when I get to Texas. November of 2001 I will go back to Dallas.