LifeinAsia - 7:25 pm on Mar 4, 2011 (gmt 0)
I can think of a few possible ways he can get the information:
1) He currently or previously worked on the site and left some sort of tracking code on the site (most likely without the knowledge or approval of the site owner).
2) He hacked the site and left some sort of tracking code.
Both are unethical, and the latter is probably illegal.