So this animal has been out of your hands for 10 months and it is your fault? I don't think the vet was honestly trying to assign blame, just trying to console the person paying the bill. "The breeder" is always someone easy to blame. If this person is trying to get money back from you, or make you pay the bill, educate her about parasite transmission, and never deal with her again (yeah, I thought that sounded like Dear Abby advice, too.).