Good question.
I can't pretend I woudn't be hurt and upset- I'd be surprised if any woman would simply accept it without feeling like that. But I would want to talk about it. I'd want to know how it came to that - what the situation was- how involved was he- what, if anything, were her actions/ feelings. I'd want to know if it was physical attraction, emotional or both. I'd want honesty. I'd want to make it clear that any involment with her would end right there and then. I'd expect him to tell her (if she was involved with him at all- depends if she knew of his attraction or not and if they'd acted on it in any way) that we had discussed it, that their relationship (whatever you want to call it) must end and make it clear that his marriage was sacred ground and not to be trodden on.
I'd also want to work out why this had got to this point- what, if anything, was lacking at home- if there were issues between us.
I'd also want to talk about what to do- how to avoid it going any further, removing himself from that situation and what to do about making sure this never arose again.
As I said, I'd be hurt- I'd be upset... maybe angry, depending on circumstances. But I'd respect the honesty and I'd be so glad that he was able to tell me. To me, that would say he trusted me and valued our marriage, even if it was under threat. I would know we had something worth saving.