files locked in public folder when sharing via file sharing
The default permissions for newly created (/copied) items allow read and write access only to the owner (whoever created it), read-only access to everyone else. The best way around this is to create an access control entry on the folder, and set it to be inherited to items created in (/copied into) it. 'Course, the Finder doesn't give you access to the inheritance controls, so you have to go to the command line:

chmod +a "group:staff allow list,add_file,search,delete,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,writesecurity,chown,file_inherit,directory_inherit" ~/Public


Note that this gives read &amp; write access to all accounts in the staff group (which basically means all user accounts); if you want a bit more control, create a group in System Preferences > Users &amp; Groups, and add just yourself and your friend's account to it, then use "group:yourgroupname" in the above command. Also, the inheritance only applies to items as they're created, so it won't apply to files and folders already in your Public folder; if you want it to apply to existing items, add chmod's -R option (chmod -R +a ...)