No, this is not a spelling mistake.
First, Quakers use Friend with a capital F to mean Quaker, and friend with a small f to mean the same as everybody else.
"Ffriend" is a word which Quakers use to refer to someone who is a Friend, and at the same time a friend. Whereas Friend (with a capital F) can refer to any Quaker, Ffriend implies a friendship above the fact that you belong to the same religious organisation. It's usually capitalised, though I "ffriend" would probably also be understood.
I've heard it pronounced fuh-friend, or sometimes fffriend. The first is generally preferred because it emphasises the double f better.