No one wakes up in the morning and says :
Man, today am going to cheat on my wife, is going to be a good day.
What happens is, you meet someone at work, gym etc. You talk to them (basic introduction as is good manners) then it becomes a habit of seeing them everyday at work, gym etc. You talk some more and get to know them as a person... before you know it you exchange phone numbers for what you tell yourself is just work related questions or we are just friends... you talk and talk some more. Before long you start to pick up on their little habits, start catching yourself paying attention to what they are wearing, how they smell, what they look like, start to really enjoy their company. Then one day there's a "slip" and she /he kisses you or you kiss them, and for some people they might stop there, but for others where this has been building up for months or even years, it's hard to stop because by then it's an emotional affair which you badly want to also get physical. It's a slipper slope.
Of course he isn't going to talk dirty to her or flirt with her while you are at home and could get up and listen in on him, but he might do that at work or wherever else they see each other.
Men and women really can't be friends, one or the other always ends up catching feelings or wanting more.
She's single, when you are single you are lonely, you seek companionship, it's only natural that you do, humans are social creatures, even introverted people want to be touched and want relationships. It's human nature and natural to want this.
It's important to note that although you probably want to tear her head off her shoulders, that she is NOT the one to blame here, it's your husband. HE is the one that made vows to you, married you, not her, she owes you nothing, HE does. He has NO business talking to a single woman at all for anything other than polite conversion, why he even has her number is beyond me and why he is talking to her for an hour late at night is also inappropriate regardless of the subject.
Tell him you'd appreciate it if he stopped all contact with her outside of work, and if it's not work they met at, then stop all contact with her period. She is single, that's trouble. Talk to him nicely, don't be on the attack, just say that you'd appreciate it if he would stop contacting her and would tell her that it's time their friendship ended because as a married man he shouldn't be getting that close with a single lady and that it's upsetting you his wife that this is happening. If this woman is any kind of good woman, she would respect his wishes and his marriage and stop talking to him.
He is not spending time with you, to instead spend that time talking to another woman whose intentions could be to be with him.
If he gets defensive about it and refuses to stop talking to her, then there's your answer... he has already developed feelings for her and isn't going to let her go.