Sneaky attack says:
Once per turn, you can inflict 1d6 extra damage on a creature you hit with an attack If you have an advantage in the attack roll.
The help action says:
If your ally attacks the target before your next turn, the first attack
The roll is made with advantage.
You can use Sneak Attack if the attack roll is made with an advantage (or an enemy of the target with 5 feet), the Help action gives that advantage, as long as the rogue is the first ally to attack the target. It's that easy.