It's simple, really.
Toxic inflicts a badly poisoned condition, which doubles the damage every turn. And Poison Heal restores HP if the Pokemon is Poisoned.
So here's my question. If a Pokemon having the Poison Heal Ability is inflicted with the Badly Poisoned condition, what happens to its health? Does it increase normally or exponentially? Or does Poison Heal have no bearing on the Badly Poisoned condition?