Accessing temporarily Another User's directories by entering his User ID and Password in Mac
There are several options:

First, set up a shared folder on your account (or both if you need both way) so that every file you'll copy there will be visible from your friend account.
You already have a public folder which can accomplish this.
Next, you can enable fast user switch from Preferences > Users &amp; Groups > Login Options. Doing so, you'll have login controls in the right side of the menu bar. You just need to login, copy the files in the public folder and logout.

Even faster, open the Terminal (Applications > Utility > Terminal), and type

su YourUsername


it will ask for password. Enter it, and you will be logged on the shell with your account. From here, browse the folders and find the files you need (cd foldername to enter a folder, cp filesource filedestination to copy). Then logout with exit. Example:

Mac:~ Jack$ su epistrephein
Password:
bash-3.2$ cd Downloads
bash-3.2$ cp picture6.jpg ~/Public/picture6.jpg
bash-3.2$ exit
Mac:~ Jack$