Even if we ignored all atmospheric conditions, we had no friction at all, the space station wasn't moving, and the aim of the sniper was perfectly on target, etc., you still couldn't pull it off.
The most accurate sniper rifle ever made, the Gepárd M1 anti-material rifle, has an accuracy of 0.7 minutes of arc. The ISS when closest to the Earth, orbits at an altitude of 409 km. This results in a target with a diameter of more than 80 meters, resulting just from the properties of the gun itself, without the bullet slowing down due to the atmosphere, etc.
And this is the most accurate you can get with a rifle. The M1 version of the rifle I used as the example, trades absolutely everything for accuracy: it has almost no moving parts, is extremely heavy, holds a single bullet, and cannot be reloaded without taking it apart. A later, more practical version of it resembles the parameters of the famous American M82.
Although the most important reason why it's impossible is described by other answers, especially by dsollen, I included this reasoning to point out, that even if all the issues raised by the other answers were non-existent, you still had a 80 meter target area as a best case.