On the champ that gets massochism fire eater(or healing in general?) works the opposite way that damage does to champs. For example if I put massochism on Nightmare and heal other champs that have fire eater with lava, Nightmare will lose AP and damage. I haven't tested this with healing in general only heals from fire eater.
So you are saying that, while Maso is up, an Eater effect is still causing the proper healing on the target, but also causes the inverse effects (sans healing) to the Maso target?