Ok, my take on this may differ from others here, but this is how I see it. Some people will claim that someone who is intoxicated is not capable of giving consent. I disagree with that up to a point. In your wife's case, I think intoxicated or not, she consented to what went on. (And I know that with her past, there may be another level to this, but I would think that with her past she would have been more apt to say no than to give in.) Nowhere in what you wrote does it indicate that this guy "forced" her to go down on him. And, if she is to be believed, she stopped before he finished, at least that is how it reads to me. No indication that he forced her to finish him, nothing to indicate that up until that point she was a willing participant.
That said, drinking too much often leads us to do things we would not otherwise do. Lets face it, an intoxicated person is not thinking straight. THIS is what I think went on with your wife. She trusted this guy to get her home, he surprised her by the kiss and without thinking clearly she didn't object. He unzipped and, possibly because she thought, based on her past, that he would force her to give him what he wanted, she initially began oral on him. As the act went on, she had time to think about what she was doing, and stopped and told him to take her home.
Good for you wife for coming to her senses. While it would have been nice to tell you about it sooner, she was likely afraid of how you would react. She was probably upset with herself and maybe even a bit embarrassed. Who knows? Now you could confront this guy or tell his fiancee or whatever, but at this point what good would that do? It won't help you forgive your wife. It won't help repair the trust between the two of you. So let it go.
As an aside, I do think this guy took advantage of your wife and is an absolute asshole for doing so. Any guy who takes advantage of an intoxicated woman who even if she consents may not be thinking clearly, is a disgusting individual. But based on what you wrote, I don't think she was sexually assaulted. She made a mistake. Hopefully you can forgive her and the two of you will grow stronger as a couple because of this.
Good luck to both of you with the counseling.