If you are located in the right folder you don't have to write the full path.
Open the terminal, type "cd " (a space at the end, no return) then click on the folder icon at the top of your finder window, hold and drag it to the terminal and the path to that folder is pasted into the terminal, then press return and you are in the right folder.
Then you can do the copy.
FYI your mail app is either not encoding MIME correctly or his mail app isn't decoding MIME correctly. MIME is perfectly compatible with resource forks, it should put the fork data in a separate part of a multi-part MIME.
The fact that some get two attachments suggests that your mail app is encoding correctly. One of them ought to be the fork.
Also: the cp trick will not only delete the fork, but will also delete HFS+ attributes like creator codes, finder flags, etc.
In order to make your attachments more 'PC friendly', I suggest you try one of these two options:
1). If using MS Entourage X, just change your attachment encoding preferences to (Windows Base-64/MIME) with Windows file name extensions.
2). Create a 5MB MS-DOS formatted Disk Image with Disk Copy. Drag your file to it and then back to your desktop, or to your mail program to add it as an attachment. [<opinion>Anything larger than 5MB should never be e-mailed anyway, so you shouldn't need a bigger disk image</opinion>