Hmmm well sure she had gotten Emolga a short while back, but since Iris was a Dragon Master in-training, she still deserved a new dragon type from my point of view. Deino just seemed perfect for her, but the writing staff apparently didn't care. ^^;
Not to mention that if Deino had been caught by Iris here, she might have fully evolved it later, and she wouldn't have even needed to get Dragonite.