Please don't question my curiosity.
They neither lose health, nor gain health. This is because Poison Heal does not have an overworld effect.
Abilities that have effects outside battle.
Also, from Bulbapedia:
Outside of battle Poison Heal has no effect outside of battle. (Poison will still have its normal effect on the Pokémon with this Ability outside of battle in Generation IV.)
Poison does not deal damage in Gens 5-7, so Poison Heal would do nothing.
The poison condition (PSN) (Japanese: 毒 poison) is a non-volatile status condition found in the Pokémon games, it causes a Pokémon to lose HP at the end of every turn, as well as outside of battle prior to Generation V.
Source