Bytebuster is right to a certain extent assuming you have a person able to learn the best method is to teach them a little bit about thngs.
The next most friendly method would be to host your own https page. If you are concerned about the files leaking enable the https server only when you are working with that person and have a password/session protected directory that you give them the password to over the phone, have it a random password that you create at that time and is long enough that normal password hacking would not break the password any time soon. Using an active page to allow upload/download like asp or php etc. Would allow short term sharing of files. The encryption could be set to the same level as what is used to process credit cards and changing the access password just before you turn it on for the session makes it so its not easily broken before your done.
The other way I could see is something like using cacert to make them a new free client certificate. With some planning and good directions you could do more or less what you were originally thinking.
The biggest thing is just document the heck out of what you want to do with the users I work with I probably hit 1 in 120 or so that I just cant explain through. Usually the problem there is they wont stop long enough to do what is needed.
If this is for end users then probably a client certificate type thing would work best. There is a reasonable amount of infastructure already in place able to use client certs. you just need a trusted repository for client certs to be managed many email programs web browsers and such already have things in place to use client certs to access encrypted items.