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Right now, what you probably most want is for ...

a.) This to have not happened.
b.) For her to go away.
c.) To blame somebody -- her, your husband, or yourself.

I firmly believe that men and women can be friends -- my two best friends over the past several years have been men and yes, I am a married woman. However, I don't keep secrets from my husband and that's what takes this relationship your husband has with this woman across the line -- he kept it a secret from you. If it was a friendship like any other, he wouldn't have needed to hide it.

An affair is the symptom of the troubled state of your marriage -- it's not necessarily the problem. Often, if you can fix the problems, the symptoms go away on their own.

I don't agree with everything on the following website and you probably won't either, but it's possible it can help you and your husband to figure out what the problems are and give you some ideas as to how to begin to fix them: Marriage Builders: Steps to Recover From An Affair/Infidelity