If he's only doing a PPL, he can use a regular B1/2 visa and for TSA purposes a private CFI can be a "flight school provider".
DEFINITELY not - that is illegal. Especially for flying!
On Friday, April 12, 2002, the Department of Justice (DOJ) issued an Interim rule through the Federal Register (Vol. 67, No.71) proposing to amend 8 CFR Parts 214 and 248. This amendment requires a change of status from any non-immigrant visitor visa (B-1 or B-2) to a student visa (F-1 of M-1) prior to pursuing a course of study, including flight training.
See :
http://www.pprune.org/professional-p...hools-usa.html