She cast a spell on me to gather gossip from Stewart, Jacob and Mafalda but when I get to Mafalda all she says is "Just smell that fresh sea air" and I can say "Bye", that's it. I can't complete this quest.
The potion only lasts a few minutes (the spell she cast) but just go back to Demelza and she will cast it again. The other npcs will then tell you their gossip.
Just go back to Demelza as often as necessary to get a new enchantment.