That's the whole philosophy behind Jeet Kune Do, learning to have no weakness. I agree that your friend should learn to fight both sides, but not until at least 2-3 montsh of one side, then have him switch for 2-3 months to the other side. If you are doing them both at the same time, its often much harder to juggle learning them both, and learn them both with correct technique. Learn northpaw, or with his left hand facing foreward first, then the other way.
Although, if you are planning on self-teaching it all, i would heavily advise against it. HEAVILY.