I think I would use this moment as a teaching moment. You now get to tell her what poly is not. It's not cheating behaviour; its loving, caring, respecting, empathizing behaviour.
Drunk or not drunk, 30 seconds is long enough to register what you are doing and stop I think. That kind of long kissing is the beginnings of some sexy bonding. Kissing is what gears me to figure out if I want to have sex with a person... if I were her, this is what I would of kissed him for... to find out if he is fuck worthy.
Friend? Ya, a phone call (not email) saying that he owes you an apology and can buy you lunch to say that to your face would be appropriate I think. I think you can be light-hearted about it, but firm about what he needs to do to make it up to you...
Once you are sitting with him and he has apologized you can tell him what it made you feel like and can tell him that while you appreciate that he is interested in your girlfriend, he needs to check that at the door when he sees her. She is off limits until such time as you have opened your relationship, or its over between you two.
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