If this was a recent event then your friend is partly to blame as the visa requirement has been widely publicised.
However all self respecting flying schools in the US should make it their duty to be abreast of their own governments regulations regarding foreign students if they undertake to train these students.
Ergo you should name the flying school as a warning to others to check these fine details before they part with a single dollar in training fees.
Safe (and legal) flying