I read somewhere quite a while ago after my dog allergic sister was forced to be next to a dog that the onus is on the allergic passenger to inform the airline in advance of said allergy so that hopefully steps can be taken to avoid offloading the allergic passenger if no remedies are available such as reseating, etc. In that case perhaps the dog and owner would be offloaded instead?
I hope someone can come along to answer if that is indeed true.