The dander is really difficult to get rid of once it has been in the house. DH's daughter has allergies and even getting rid of a cat didn't help for a long time.
People can develop allergies to anything at any time. I feel awful that you and she are having to deal with this.
Maybe she can try Nasalcrom ....it is over the counter and it is a lifesaver for me during Birch pollen season.
It is not systemic and I find relief within 24-48 hours, but it says to use it about a week before being exposed.
Thoreau said, 'A man is rich in proportion to the things he can leave alone.'