Our paladin has the Good devotion feat active. My swordsage uses his Shadow Garrotte within 30 ft.of the paladin and hits a devil with DR 10/good. Shadow Garrotte is a supernatural ability but it's damage is normal damage as I see it. Does the attack bypass the damage reduction? Would the devil also have to be within 30 ft.of the paladin for the garrote damage to bypass the DR ?
No, Shadow Garrote damage isn't weapon damage, so DR of the devil doesn't affect it.
You may be interested in weapon attacks as well. In order for your weapon attack to be able to pierce devil's DR, you should be within 30' of the Paladin. Where the devil stands is irrelevant; the feat targets weapons not enemies. Your weapon is just treated as good-aligned in the radius of Good Devotion.