Mate's girlfriend came on to me

As the title really! I was in a club and she just came up to me and gave me a kiss on the lips. It was pretty sensual and she held it for far too long. I started to respond but quickly realised what was happening and stopped. A mate of hers saw and came and got her away. I think tongues were pretty close. We were both fairly wrecked, but I obv feel bad as she has been with my mate for years. But we aren't that close. Any ideas what on earth she was thinking? Do I tell my mate? How do I react around her? I'm awful in awkward situations.

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