This took a lot of effort to try and test BUT near as I can see, the “heals to 50%” occurs regardless of the sand empire healing debuff.
(I.e. if Yunan or Rana have the healing debuff on a hero, Raffaele will still take them back to 50% HP)
Then Raffaeles Healing Boost buff (same as the one that Triton has) overwrites the healing debuff so further Heal Over Time is unaffected.