The friends tab is a part of the Event screen
In this tab are listed all members of your event. For each of them you can see balance. Red negative amount means that your friend must give back money, green positive amount means that your friend must receive money. To know who owes to whom take a look to the Who owes to whom tab.
As you can see in the screenshot, you can group the friends (by family, affinities, etc...), the groups will be shown instead of friends in the Who owes to whom tab. To group friends, you can edit the friend and select a group (Friend edition)
In this menu you can:
If your friend is involved in operations you cannot delete him without impacting the calculation. So in this case you will be prompted to transfer all your friend operations to another friend.
Just select who should support operation for him.
Example: If your friend has a debt and will never pay it, so first it is not a real friend :), and if you want to dispatch the debt to several persons: first add a refund operation dispatch de debt to who you want and to decrease the balance of your non-friend to 0, then delete him (select anybody to merge with, in order to not loose the history of operations)