TL;DR:
Open AirRoster → Friends → Friend List (top right) → All Friends → Remove. The selected friend will be removed from your list.
Launch the AirRoster app on your device and ensure you are logged in.
On the Friends page, tap the Friend List button in the top-right corner.
In the Friend List window, select All Friends to view your complete list.
Find the friend you wish to remove and tap Remove.
The friend will be removed immediately and will no longer appear in your Friends list or shared roster view.
If you experience any issues removing a friend, please contact Wingman Support:
support@wingman.zohodesk.com