Don’t exploit friends, but don’t do freebies either. Break even — or else you lose both business and friendship.

When doing business with acquaintances who drop by, you shouldn't cheat them, but you also shouldn't do it for free. Breaking even is best.
Otherwise, the more you do business, the fewer friends you'll have.
If you lose money every time and they still keep showing up, it proves they don't value you as a friend.
If you make more than usual this time, that's even clearer—you've never really considered them a friend. In that case, at least it's straightforward.
But the former situation is much more painful.