You're probably not killing the clutch, unless he's doing hole-shots or something... if the clutch can't survive a few stalls, how could it ever hold up to racing?
A related story...
Within the first week of owning my car I volunteered to 3 separate manual-tranny-driving friends that they try the Boxster out. Well...
over 20 stalls later, several in
very interesting traffic situations (no kidding - all my self control skills were well challenged that week
), I changed to a new policy - nobody drives my Boxster but the person I share my bed with. And she
doesn't want to drive it, being worried she'll break something... she's more than happy to soak in the sun and let me have the driving fun.
Given this, I do appreciate your generosity in giving a 19 yr old stick-novice the chance to drive your S! Geez, after all, my Dad didn't really even want me driving the family's 1968 Buick Riviera!