My warrior was my last toon that had NOT accomplished a successful Riverweed forage. I was about to call it a session and log him out when on a hunch I clicked FORAGE on my druid and sure nuff, got a Riverweed. I was about to bag it when I accidentally opened a trade window with the warrior. Oh well, I figured, doesn't matter who holds it so I completed the trade. THIS GAVE THE WARRIOR CREDIT FOR A FORAGE OF RIVERWEED IN THIS QUEST! Evidently the toon does not have to accomplish the actual forage, just acquire a sample of this item. Which must mean you can even just buy it from another player.
/boggle