I agree with Sleepy538. Could you actually talk to the girl’s mom and get the real story? If it’s an allergy, suggest they switch the hay (IF they’re even giving hay).
Like I said tho, talk to the mom (or dad, or whoever is in charge over there). We have enough experience talking to teenagers on this site, it can be like a brick wall trying to get through to them!
Oftentimes, rabbits (and other pets!) are given up because of “allergies”. BUT…”allergies” seem to be the catch-all term for “We don’t want this rabbit anymore but we sound like crappy people if we say that so let’s blame it on a medical issue.”
Do you know if this was an Easter bunny? I’m not sure where you’re located, but I’m in Ohio and the pollen count is outrageous this year. Even people who DON’T typically experience seasonal allergies are MISERABLE!!! So it could be possible that she’s having bad seasonal allergies and it’s being blamed on the bunny since the bunny is “new”. Just a thought!
I think you’re doing the right thing by standing your ground. You have to do what you can for your current pets! If you can’t do it, you can’t do it, period. Maybe offer assistance for them to go through a shelter or rehome on their own?