Befriend Emma enough and you'll have a new option to invite her over. Do this and feed your character in front of her. After doing it a few times, you'll unlock new dialogues and events with her. Keep it up and she'll eventually become your character's feeder.
Depends on how you decide to feed your character. If your character eats a lot of food on an already full belly, it will have much more effect on Emma. If you just eat a single watermelon to hit max capacity and call it a day, it'll take longer.
