The only rule is that there are no rules -- just watch Ken Perlman play clawhammer using the 5th string fretted for notes instead of playing it as a drone and you will realize that all bets are off regarding how you "ought" to play.

Seeger's basic strum involved an up-pick, but he then described frailing as it ususually is played, with a down stroke using the back of the nail. Or at least that's what I remember learning from his book.

Best bet is to send that son's friend out to watch as many players as he can find. At 16 years old, he'll work out his own way of stroking the world soon enough.