Normally all my expertise in the mechanics of LoL would discount this as NO before it even began as a question. However my friend claims MULTIPLE instances of activating his stealth as Kha'zix WHILE the enemy turret is focusing him, and having the turret STOP attacking him.
Kha'Zix's stealth is no different from any other, and towers can see him just fine.
It will currently also, however, sometimes cause towers to lose focus and attack other targets. This may be a bug.
Kha'Zix's ult reduces damage while stealthed (even when he's visible). Reduced turret damage might look like he's not being shot at.
Was Kha'Zix skirting the edge of the tower? The turret's visible range is just a hair larger than its firing range. If Kha'zix was firing his skill shot from the edge, he would not provoke tower aggro (because the tower couldn't reach him!)
Your friend is fallible. Perhaps your friend is merely gushing about the new hero! Just because he thinks he's avoiding tower shots doesn't mean he actually is.