Ants on a Board

Your friend challenges you to a game with a square board, a round glass, and 51 ants.

This the how the game works: your opponent will first get to place the 51 ants anywhere on the square (which has side length 1), and they will stay perfectly still. Then, you will get to place the round glass (which has radius 1/7) anywhere on the square. If you can capture at least 3 ants under your glass, you win. If not, your friend wins.

After thinking about it for a bit, you agree to play. Your friend laughs! He says that he can always win this game. Is he right?

Source: The green book