In the interest of completeness, and perhaps fairness, I'll now tackle the issue of what a guy means when he uses the word.
I think it's much more straightforward. There's a definition that's never used.
See, a guy will never, ever, ever, use the word friend to describe a girl unless either (a) He is not sleeping with her and, for some reason, wants to make that point perfectly clear, or (b) He's not interested in her physically and wants to make that point perfectly clear.
For example, I often refer to HatGirl as my friend. I use that word intentionally, because HatGirl is extremely engaged. I don't want anyone to ever jump to the wrong conclusion about me and HatGirl. So, reason "a" above definitely applies.
In another case, referring to another girl, I might use the word friend to stress the point that I'm not even interested in her in that way. This is a term I might use when referring to, say, Roseanne Barr* or someone of similar appearance.
The difference between these two meanings is a subtle one. It's usually taken from context. If, for example, you see me standing next to HatGirl or LaptopGirl or any other girl that's not a big fat hog, and I refer to her as my friend, well then it's fairly obvious that reason "a" above applies. The use of the word friend in this case isn't meant to either confirm or deny the presence of physical attraction - it's only meant to make it clear that there's nothing currently going on. Usually to spare the girl's reputation.
So, I'm really rambling with these two entries. I really did have a point to make. Or a point to illustrate.
But now I've got to work some more. Don't hold your breath. I may decide that this is too stupid, even for this venue.
* - I'm sure that Roseanne is a wonderful person. I only used her as an example. No offense intended.