Does the car have a traction problem? If so, I say go for some drag radials, real sticky Nitto's, but I really don't see a need for slicks just yet. If he doesn't have a traction problem, then I'd just stick with those tires, and if he races often, just start upgrading the engine, etc, and go from there to get some lower times. Hope that helped.
If all he does is smoke up street tires, he will go faster with slicks. With a 15 sec. car he should be fine with street tires. If not, he should work on the car setup a little. Play around a little, and get used to it. It will pay off if he puts more power to it. With my 70 coupe, I did not need slicks till I started hitting 13, and 12's.
Also, just wanted to add.....Pete(69RedMach1) has a 69 Mach with a mildy built 351W, and he's running in the 13's without nitrous. With it, he's running in the 12's. /forums/images/icons/shocked.gif You might want to PM him and get some info on his buildup. HTH.