Lulu can cast Help, Pix! on allies and enemies. If she casts it on an ally, Pix's magic missile barrage attack triggers on the ally's auto attacks. If an enemy is killed as a result of this damage (rather than the triggering auto attack) is Lulu granted the kill or is her ally granted the kill?
Based on this thread the kill is awarded to the Ally buffed. It works the same as Leonas passive. I have seen a few LuLu games and also the answer in question in that thread has +10 so one can infer they are correct.
There was a Riot post that states the Ally Buffed gets the kill, I have however experienced a situation where I buffed the jungler and they were after a buff that I got (I was not actively doing any damage). We assume pix got the final hit and the buff itself transferred to me instead of the jungler so beware of that possible hiccup in your travels :)