Quakers use the word Friend to mean Quaker. This use stems from the official name of the Quaker religion, The Religious Society of Friends. Friends tend to use "Friends" rather than "Quaker" when they are among Friends, since they are more likely to be understood.
Note that when Quakers use the word "Friend" in this sense when they are writing it down, they almost always capitalise it, to distinguish it from its more common meaning. When both meanings are intended at once, Ffriend is used.