One cannot light the Chanuka lights before p'lag hamincha (Shulchan Aruch 672:1), not even on Friday evening (Mishna B'rura 679:2). If he did so, his lighting is invalid (Shaar Hatziyun 672:4).
P'lag hamincha is defined (MB 672:3, Kitzur Shulchan Aruch 139:10) as a point in time one and one-quarter season-based "hours" before tzes hakochavim (when stars come out). That is, divide the length of the day into twelve, choose one of those divisions as an "hour", and subtract one and a quarter such "hours" from tzes hakochavim: that's p'lag hamincha, the earliest time to light the Chanuka lights.
Now, the practice in the United States, at least, is this: No matter how late you consider the day to start and end for other purposes, you consider sunset as the start of ben hash'mashos with respect to doing m'lacha on Shabas. To put it more simply, no one lights a flame after sunset on Friday.
But some hold tzes hakochavim is quite late! For example, consider Seattle. During Chanuka this year, its day length (sunrise to sunset) is about 8 hours, 25 minutes. Even if we add 72 minutes to the start and end of that for twilight, we have a 10:49-long day; thus, even if we count "hours" including twilight, an "hour" is only 54 minutes and 5 seconds. People who hold of a tzes hakochavim that's more than 67.5 minutes after sunset have a p'lag hamincha that's after sunset.
When do they light on Friday of Chanuka, and on what basis?