To remove a friend, first go to your friends list by hovering over 'social' on the left menu and clicking 'friends' or, click here: https://www.chess.com/home/friends
From your friend's page, find the friend you want to remove. On the right side of their image, click on the 'remove friend' icon:
Then click 'yes' in the text box that appears. This will remove the member from your friends list!
Mobile
From within a game you can remove a friend by tapping on their avatar and selecting "remove friend"
Or, you can always locate them in your friends list by tapping on 'more' in the bottom toolbar, then tapping 'friends':
This will take you to a list of all your friends! Tap the three lines icon to the right of the friend you wish to remove, and choose 'remove friend':