Sharing and permission

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 & 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 & 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 ...)

Looks like you're having this problem. You will need to install the real Samba, not Apple's version of SMB. It might be easier to put the SMB share on one of your Windows machines, though I don't know how that will go on Windows 8.