Politely ask her to stop. Let her know that this kind of conversation makes you uncomfortable. For example, you could say 'I'm not comfortable with this topic, can we talk about something else?'
One should firmly and clearly express that such talk is unacceptable. Set boundaries and communicate the importance of loyalty in the relationship.
If you're uncomfortable, you should communicate your discomfort clearly. Say something like 'Honey, I don't think these stories are appropriate for us to share right now.'
Politely stop her and change the topic. For example, you can say 'This topic makes me uncomfortable. Let's talk about something else.'
One should firmly oppose and ask the person sharing to stop. It's not something that should be talked about.
A good response could be to engage in a positive conversation about it. Maybe ask some gentle questions to understand her fantasies better, like 'What made you think of this fantasy?' But at the same time, he should respect boundaries and not push her too far if she seems uncomfortable. He can also share some of his own non - sexual but intimate thoughts to create a more equal exchange of feelings.
He should first listen attentively and non - judgmentally. Let her know that she can trust him with such personal thoughts. Then, he can share his own feelings about it in a respectful way, whether he's comfortable with the ideas or has some concerns.
You can start by showing interest. For example, say something like 'That sounds really interesting. Tell me more.'
I would react with disgust and disbelief. I would try to stop the person from sharing such a story because it is not only inappropriate but also harmful. I might also suggest that if there is a real problem in the relationship described, it should be dealt with privately and through proper channels, not by spreading such sordid tales.
Politely but firmly ask her to stop. For example, you could say 'This is not the right time or place for such stories.'
Politely change the subject. Maybe start talking about the beautiful scenery during the vacation instead of the sexual aspect. Say something like 'You know, that beach we went to was so beautiful, let's talk about that.' This way, you can avoid the discomfort of the inappropriate topic without making her feel bad.