As jffurlan says, they can simply install it from the command line, assuming the unit they have has support for enabling Development Mode in the settings.
They'd need to do that, including setting the password to be used. They need to establish a network connection, and note what it is.
Then on a computer (PC, Mac, Linux), download and install the AIR SDK from Adobe, and the PlayBook SDK, following the usual instructions for doing that. (The AIR SDK is needed merely to satisfy the PlayBook SDK installer, even though they aren't going to be using it.)
Once it's installed, they will have a "blackberry-deploy" file somewhere in the bin/ folder of the SDK installation. They'd need to put your .bar file someplace accessible, then (with that utility accessible via the PATH) do the following with appropriate substitutions based on the info they gathered above:
blackberry-deploy -installApp -launchApp -device X.X.X.X -password YYYY -package YourApp.bar
Obviously you'll need the person who does this to be fairly computer literate, or you may end up spending hours walking them through the process.
