After completing Festival of Greed and waiting at least 1 day after completing Sick Workers, you can find the quest either in the Adventurer's Guild or by talking directly to the farmer girl in Riverflow.
After talking to the farmer girl, go to the alchemist in Trademond. He will indicate you the formula peddler (to the left of the Boar Hut) as your next objective. Go knock to the door. From now on, you can get the formula you want by paying 2000 G or accepting a lewd action (3 base minimum lewdness).
Once you have the formula, bring it to the alchemist with an emerald leaf. If you already lost all the emerald leaves you can find, you can still buy one in Nephlune. Once you brought the material and the formula to the alchemist, wait 1 day and talk to him again. You unlock anti-magic coating at his shop and obtain a free sample.
When you talk to the farmer girl with an anti-magic coating in your inventory, you automatically give it to her. After that, if you have pushed Rose in the mental world (entered the second chamber), you'll get a choice between 3 options.
Once you gave the coating, go to report to the guild clerk to complete the quest.