Putting the files into an obscurely named and unlinked directory of a
website will normally be as good as necessary. The sender still has to
mess with ftp, unless he has a web-based uploading system at his
disposal (see fotopic.net for an example user interface).
Okay, so what are several ways to share files with a friend, where you
don't share any accounts or passwords, and where only your friend will
be able to access them?