Healer Only Triggers in battle, so the only way to trigger this scenario is a triple battle. However:
At the end of each turn, Healer has a 30% chance of curing an adjacent ally's status condition. If the Pokémon has multiple adjacent allies with status conditions, it has an independent 30% chance of curing each one, and can cure more than one in the same turn. If it cures a status condition, it does so before burn or poison damage is dealt on that turn.
Hope this helps