Inspired by Can I use deflect missile if I get an ally to shoot me? and Can a monk catch and throw an Arcane Archer's active Arcane Shot back at them with Deflect Missiles?, can a (friendly) Arcane Archer choose not to activate their Arcane Shot when it hits their ally, but then activate it when it hits the new target?
Once per turn when you fire an arrow from a shortbow or longbow as part of the Attack action, you can apply one of your Arcane Shot options to that arrow. You decide to use the option when the arrow hits a creature, unless the option doesn’t involve an attack roll.
So in this scenario, Monica the Monk runs on ahead, in a position where she can see Edwin the enemy mage. Archie the Archer does not have line of sight to the enemy caster. He fires his arrow at Monica the Monk, who uses her reaction to deflect it successfully.
If you catch a missile in this way, you can spend 1 ki point to make a ranged attack with the weapon or piece of ammunition you just caught, as part of the same reaction.
Let's say the arrow (thrown as an attack by Monica the Monk) now hits Edwin the enemy mage; can Archie the Arcane Archer then activate the Arcane Shot to try and affect Edwin the enemy mage with it?