One common theme is the exploration of their relationship. Given their different personalities in the original work, fanfictions often focus on how they bridge the gap between them, like how Giyuu's stoicism and Shinobu's more cheerful yet calculating nature interact.
He should listen attentively. Look into her eyes, nod his head to show he's following along, and make small comments like 'Really?' or 'That's amazing'.
You could also try to shift the conversation to a more positive and relationship - building topic. For example, 'Rather than talking about that, let's talk about our next date or something really special we can do together.' This way, you avoid getting into an uncomfortable argument and focus on the good parts of your relationship.
Naruto would probably be initially shocked. He's a very loyal person, so he'd have a hard time believing it at first. Then he'd likely be very angry. He has a temper when it comes to things that are unjust, and this would be a huge betrayal. He might also feel really sad and disappointed in himself for not seeing the signs earlier.
First, take a deep breath and try to stay calm. If it makes you uncomfortable, be honest and tell her. You could say something like 'I'm not okay with this story, it makes me feel uneasy.'