Zarya can definitely counter a Reinhardt on a 1v1 duel, assuming Zarya has a bubble for herself. As long as Zarya stays clear of fire strike and Reinhardt's charge, you can easily beat Reinhardt. If the Reinhardt has Earthshatter available (assuming that you've been tracking ultimates), remember to react accordingly once you hear him scream "Hammer Down." and bubble yourself as this will protect you from the stun and the damage of Earthshatter.