Thank you for that additional info.
It's hard to say what her motivation is. Perhaps he's just easy for her to talk to, perhaps you weren't talking to her enough, she could be bored, or he could be making her feel that she is of some use or "needed" by him. Regardless of the reason, lying is never acceptable. The botXXXXX XXXXXne is that she is looking for something or lacking something, the good news is that it may have nothing to do with you, and everything to do with here. You'll want to sit down with her again and let her know how her behavior really bothered you, but what particularly bothered you was the fact that she lied to you.
There's two choices here, she can either come fully clean and let you know what's going on in her head and what the two of you can do together to make things right, or she may need to seek therapy to figure out why she would jeopardize her marriage over something so insignificant. Couples therapy is probably the best way to go, but that's really up to you.
As for whether or not anything more is going on, I would ask you, what is her track record? Has she lied to you in the past, has she cheated on you in the past? The best indication of future behavior is past behavior, and if she's been honest and straightforward with you up to this point, I would say to give her the benefit of the doubt, providing she is willing to be upfront with you from this point forward.
I welcome your thoughts, let me know if you want to talk more.