John, honestly I dont think I can make this any clearer than I already have, but i will try. What she did was in no way, shape, or form, just a "dinner" with a friend. It was a date to her and to him, and the only reason you think any differently is because of what SHE has told you and because that is what you WANT to believe. Not by any other actions. In fact the only action in this whole scenario is that of a full-fledged date.
She meets him at the high school thing. They talk. She gives him her number. He calls-asks her out. She says no, but how about dinner. He accepts. She doesnt tell him about you (I mean cmon, if she was head over heals in love with you and didnt want to risk losing you, wouldn't she have bragged about you to him at the reunion?, I know I would). He accepts the counter offer, she then cooks him dinner, they break bread-drink wine-talk-hell they had sex for all you know. And thats the thing-you dont know. She has already shown some shady behavior in this whole thing, why should her saying that it was just a friend thing to get him acquainted into town be the whole truth? I could see if they were old high school friends, but they weren't. Dont you think it odd that he then comes over for dinner if he barely knows her? I do. All you know of what happened is what she has told you. Nothing else.
I told you what happened with my ex finace'. The exact same thing happened. I was in love with her, therefore WANTED to believe everything she said was the truth, even though a little voice in the back of my head said something is fishy. She said, "I have nothing to worry about, were just friends". Right. A month later she moves in with the guy. If you did the same thing that night with a girl, would you tell your gf that you had feelings for this new girl? Of course not. You would hide things until you knew for sure what you were going to do-either stay with your gf or go with the new girl. Your not going to burn bridges until the t's are crossed and the i's are dotted.
What she did was soooooooooooo inappropriate and disrepectful towards you. Take it from a guy who has been through this, this girl is not telling the truth. Nobody invites a person over for dinner on a friday night when they already are involved with someone unless there is SOME KIND of romantic interest involved. It just doesnt happen.
IF you want to stay with this girl, you need to tell her you need some time and space for a while to think about what she did and if she is the type of girl you really want to be with. This will either scare the ****e out of her and she will never do something like that again when she realizes what she could be losing, or she will in the back of her head say thats cool-thats what i wanted all along. Either way you win and come out looking like a champ. I cant stress this enough------------THIS IS A PERFECT OPP FOR YOU TO TAKE TOTAL CONTROL OF THIS SITUATION AND RELATIONSHIP IF YOU PLAY YOUR CARDS RIGHT. But to do so you must listen to me exactly. I can guide you though this and you will be thanking me for the rest of your life. But you must do as I say. Right now she has total control of the relationship, I think deep down you know this. Let me help you and I can gaurantee you will come out a winner, its up to you.