One way to look at it is in terms of the difference between talking about feelings and acting on feelings.
In general it will feel really good for the client to be listened to with empathy and treated with kindness and experiencing everything she wants to talk about being welcomed, validated and held. This can lead to the client having all kinds of positive feelings for the therapist, including some that maybe she thinks she should be feeling for her husband. If she brings those feelings to therapy, and the therapist welcomes, validates and holds them as well, then from her perspective it might start to feel like an emotional affair.
The therapist will also feel things. It may be similar to what the client is feeling or totally different, but he is going to feel something. There may be instances in which it’s appropriate and helpful for him to share some of what he’s feeling with the client, but for the most part, he will need to take his own feelings elsewhere to work through as needed.
Above all, he must not act on his feelings with the client, nor must he encourage or allow the client to act on her feelings with him. The client can talk about her feelings all she wants. They can both feel their feelings. As long as that’s as far as it goes, it’s a healthy therapeutic relationship.
But the moment feelings get acted on and that becomes the norm, especially when the therapist acts on his feelings or encourages his client to act on her feelings, then the relationship can cross over into being an emotional affair, especially if romantic-type feelings are mutual.
In this case it’s all about the therapeutic frame and how well the therapist upholds it.