Lavica, this sounds like a one-time event, and while you certainly feel betrayed, it is definitely possible to move past it. it sounds like he didn't search her out, but she took advantage of him.
I recommend that you read "Not Just Friends" by Dr Shirley Glass. it will give you tools for dealing with this, especially since your husband is sorry and remorseful about it. you might want to have him read it too, but you read it first.
good luck and let us know if you need further help or encouragement!
