very good that you read the book. actually there are examples there of a one-night stand. and even though a one-night stand can also be a hurtful betrayal, you can remind yourself that it just happened once, your husband didn't initiate it, and basically she took advantage of him. you said it before, it was like he was raped (and yes that can happen even to men). your husband is remorseful and wants to repair the damage. so now you both have a job to do. you should read the parts of "Not Just Friends" and also "Divorce Remedy" about recovering from infidelity. it doesn't matter if your case is different from the ones described - actually the cases described in the books are even more difficult to get over because those are examples of longer relationships where the husband really did have feelings for the OW, not like your case where she took advantage of him and it just happened once. a few guidelines: 1) at first you will go through a painful emotional time but this will also pass; 2) use Michelle's "stop sign" technique when you are ready to put it out of your mind; 3) you can forgive even if you can't forget.
also about "forgetting" - it might not seem like it now, but time does heal. as time goes on and you see that your husband is being faithful to you, little by little this bad experience will become fainter. you *will* be able to get over it. read the books, follow the suggestions, and most of all give it time.
as for OW - don't even give her a second thought. you write that she "is having fun outside, no worries, no regrets". what do you care about her? your husband chose *you*. and she is the outsider that he doesn't want. who has the last laugh?
