I will admit I have come into this conversation late, and only read the first and last pages... That said, I have a suggestion.
If the young lady is still gone, and you are still unhappy about the adition of the new ferrett to your home, (understandably so) Find him a new home quick. a good permanent home. That sends a pretty clear message to the young lady that you are not a dumping or holding ground for things or animals. as long as she can get you to care for them, there will be other animals on down the line.
Ferretts are wonderful intelligent and sensitive creatures that really need a lot of play time and attention, they go a little batty locked up and alone. I hope you havent spent too much of your chicken money on the lil bugger yet, if so, ask for some of it back from the adopters, Most people are ok with that!
Sounds like you have your hands full, and my heart goes out to you. If it is at all possible, try to get some professional counseling, it is not as pricey as you might think, and unbelievably helpful! Sometimes we need a sounding board for these issues, and this young lady has more than her share. The more knowledge you can gather, the better the situation will become in the long run. And remember, her actions are not about you, they are totally and completly about her! You and your feelings are clearly the last things on her mind.
Good Luck!