It has to do with Chet and that marketing lady but after you complete their quests Beth will eventually start gaining weight again.
So the fairy does nothing to her like Stephanie and Jaime did
It helps with unlocking her last stage for all out. You have to complete Chet and Yuna's quests for Beth to go all out.
What tanya quest are you referring to yuna?
Yes, Yuna
There is no direct interaction between Beth and the fairy, if that is your actual question.
