Can Facebreaker stun units when you are asked to help in a showdown that you are not participating in, or do you need a friendly unit in order to cast?

Ruling: You need a friendly unit to cast Facebreaker. Spells need all their targets in order to be announced and go on the chain.

Nuances:

• You cannot cast Facebreaker when asked to help in a showdown if you are not participating in it, because you would not have a friendly unit as a valid target.