Well, obviously it needs to be downloaded from somewhere, so ultimately you'll need an internet connection at some point.
But basically, what you do is right-click on the link for the encoder (or whatever other plugin you want) and choose Save As... (this is assuming you're using Internet Explorer - if not, then you'll have to adjust these steps accordingly). You should wind up with an .mjp file saved somewhere on your PC. Open that file up in Notepad and copy the URL line. Then paste that URL into your browser and you'll end up downloading the actual plugin's ZIP file. Make sure you put the ZIP file in the same folder as the MJP file. Now, edit the MJP file so that the URL line lists only the filename (plugin.zip for instance). Then all you have to do is copy both of these files to your brother's PC and double click the MJP file. MJ should install the plugin the same way it would if it were downloading it. I'd recommend storing the MJP and ZIP files together somewhere for future installations, so you don't have to go through all of this ever again.