It looks to me like you do it by going to the "aspect" the friend is in (there's a list on the left side of the screen) and choosing the pencil icon that allows you to edit that aspect. Then the screen that shows you all the people in that aspect allows you to delete anyone you select using a button at the bottom left.

Now, if a person is in more than one aspect, I'm guessing you have to delete them from each one.