To Unfriend someone, go to the Message section > open your Match discussion > tap the menu icon in the top right corner > Tap Unfriend.
You can also Unfriend a Match from the Friends list. Go to the Message section > scroll your Friend list until you find your Match > tap the menu icon in the top right corner > Tap Unfriend.
When you Unfriend someone, you will also disappear from their friend and Match lists, and they will disappear from yours.
Note: Unfriend is a permanent action and can not be canceled.