you can email your friend a "magnet" link (like a web link, but for LimeWire/gnutella files).

right-click (control-click) on the file in the LimeWire Library, and choose "magnet" details. Copy the link and email it to your friend.

When your friend clicks the link, it should fire up his or her limewire, and begin connecting and downloading from you (if you have open slots available).

Cheers