It wouldn’t matter how intoxicated I was, I would never be inclined to “get frisky” with my best friend or any other female or any other male that wasn’t my SO. It’s just not in my make-up. Just like I would never drink and drive. It’s a decision that has been made way ahead of time (years ahead of time), so things like that don’t ever happen.
Things don’t “just happen.” People use intoxication to justify things happening. So you need to figure out why this happened. It’s likely that you’ve been thinking about experimenting with a woman, or this particular female friend, for a long time and probably never said it out loud. In my opinion, both of you were cheating, not good. How would either of you feel if your male boyfriend and her male husband hooked up with each other? If either of you is okay with that prospect, then it seems like you all don’t really believe in monogamy. Some people don’t, but you need to figure out if you and your friend and your SO’s do.
I would talk to the best friend first, apologize and say that you were drunk and it will never happen again. Then let her talk. See what she says. She will either say the same thing, or she might say that she liked it and it’s no problem, or she might say that it was great and she hopes you feel the same way, or she might not want to talk to you at all (right now, or possibly ever) because she’s feeling guilty and foolish and embarrassed too. The answer that your friend gives, should dictacte what you do next.
Don’t be so quick to tell your boyfriend, unless your feel very confident that he won’t be hurt by this news. If you and your friend decide that the whole situation was a big mistake and you can both keep your mouths shut about it (and no bystanders are likely to blab) then let it go, don’t ever let it happen again, and be a better more upstanding girlfriend.
You can also decide to let your boyfriend know what happened, but be prepared for his reaction. He might be hurt (that you cheated) or embarrassed (that this all happened while you were drunk and in public) or he might be titillated and want you to hook up with the friend or other women again. He will either think you cheated and betrayed him or he will be turned on and want you to do it again. But if you do tell him, the subject is likely to be brought up many more times in your future. Think about that.
In the meantime, you should really think about the fact that you have some sort of a drinking problem. No one should ever let themselves get to the point that they do something (that in their sober state) they would think is wrong, or embarrassing, or dangerous or hurtful to someone else. Think about things ahead of time (years ahead of time) and set limits for yourself. Then decide what kind of a relationship you really want and conduct your life in that manner.