Although I'm all for honesty in a relationship I would actually do as your friend asks of you in this situation.
They're obviously very close, and have been for some time. Intimacy and trust between them isn't much of an issue. As you said, she was drunk and therefore her judgement was clouded. However, she did make the conscious decision not to take the situation any further. There's a difference between kissing someone when you're drunk, and sleeping with him or her.
As a male, I could cope with it if my girlfriend did that, but I probably wouldn't want to know about it if possible because it would cause some emotional damage on mine and her part. The end result for them will be that they stay together if they're as close as you make out. After all, it was only a kiss. This way, only the female friend has any kind of punishment by living with the guilt for a while. The other option would result in leaving them both hurt.
Obviously, if it happens again take some more drastic action. Do tell her you're not comfy lying for her.