How viable is giving Warding Breath to Nowi?

yea it'll work

Works great especially when paired with DD3

Warding breath is viable, because you are using it mainly for the cooldown boost anyway. To get higher arena score, change your support skill to a dual rally and give her a more SP expensive C skill (dual spur skills, drives, tactics, smokes and so on). Also, since you would be using warding breath, you can ditch the heavy blade seal.

Warding breath is good; I would personally play it with close def to bolster her def (which will be targeted mainly by melee, but you might need to battle an occasional Breath of Fog and it'll be a nice boost. She wins without CD but it's nice to have). Others are suggesting distant def, which also makes a lot of sense to really bulk up against mages. I prefer Fae to be bulking up against mages because red mages are uncommon and green mages are fairly common, but it's not a bad move on Nowi.