I don't know why that wouldn't work for KEI offhand. Should work fine.
Also, if you want the NPC to cast the spell, you want to use this instead:
Code:
$npc->CastSpell(2570, $ent, 10, -1, -1);
Though, doing that will limit the NPC to only being able to cast on 1 target at a time. That is why I recommended SpellFinished().