...It reads "When you suffer damage, the monster that inflicted the damage suffers one as well." Since it says "one" rather than "that much damage," I'd assume it means they lose 1 heart regardless of how many points of damage they inflicted.
Correct. Reflective Hide has a "triggered" effect. The trigger is "when you suffer damage" rather than "for each point of damage you suffer" or some such variation.
When damage is dealt, Reflective Hide simply asks, "Did you suffer damage? Yes or no?". If the answer is "Yes", then Reflective Hide deals one damage to the monster that damaged you.
Damage is all resolved simultaneously for each effect. When an attacking monster rolls 3 claws, they aren't dealing 1 point of damage three separate times. They are dealing 3 points of damage one time.
Hope that helps.