Hi, I tried with my party member partner on every single PVP mode and it didn't work. She could not heal me or join the aggression in any way, so I'm guessing the only way of doing so is without being in a party, which means you cannot heal your party member partner in this single instance. Test were driven in Quintera (Open PvP) and this is a screenshot of the testing phase being executed:
-My friend with the PK lost PZ on every single stage to try if it would trigger different if my friend in the party would have a different PvP mode activated when the agression was triggered (Yellow hand, White hand, Red fist) it didn't work.
-Although my party member friend is not a Druid as you said in your question, we used UHs for the testing, which would've worked the same way if any.