I think if you have been in the LS long enough something "like" this is bound to happen. Now let me clarify, I don't mean your partner banging someone else and not knowing it wasn't you. What I mean is rules or boundaries getting broken. It just happens and I think it probably happens to everyone at one time or another. These are the learning and growing spots in your life. It is how you communicate and deal with the busted rules that show you how strong your relationship really is.
I agree with Susan, I strongly believe that she meant no harm in doing what she did. She got carried away and know feels bad about it. Like some have said maybe your approach to the situation wasn't the best, again it happens. Maybe instead of "yelling" like you did, maybe you could have moved through the crowd until you where in front of or near her head. Bent down and lovingly gave her a very passionate kiss. This could have let her know you where "ok" with what was going on, and would have had the side benefit of letting realize that you where not the man inside her. If she then started to panic, you where close enough to help her out directly and be a calming influence. Just a thought.
At any rate, I hope all has worked out for you and your SO. I know what it is like as I have broken most of our rules that we once had at one time or another AND Mrs Van has broken many also. Like I said, it happens, but it is how you deal with them that really matters. Talk to your SO, then talk some more and then when you think you have it all talked out, you might just want to talk about it again.
-Van