Ouch, your pal was really bloody drunk if he did this. He might have done it instinctively because he was a drunk and, if his best friend is a girl, that probably loved her a while ago and got friend-zoned somewhere down the line... (What? It does often happen).
When he was drunk, he maybe got some kind of cognitive bias (more precisely, a <url=http://en.wikipedia.org/wiki/Wishful_thinking>wishful thinking bias) caused by the alcohol corrupting his train of thoughts. To speak clearly: his heavy drinking made him think she'd be okay with this, or that he secretly liked him, an assumption which, of course, would sound completely ridiculous to him when sober.
However, I want to clear something out for the rest of this discussion: what is the eye colour of this friend of yours? If it's blue, he's clearly in the wrong, as blue-eyed people can absorb a higher amount of alcohol without getting intoxicated to the point their decisions are completely fucked, as in they wouldn't do it when sober, which would confirm the statement "he abused/took advantage of her".