How can I prevent a Rover sitter from continuing to provide dog boarding to other dogs if their own dog has a highly contagious virus?

During Christmas, I left my dog with a Rover's dog sitter who provided excellent care, and I even left her a 5-star positive review because I always respect and appreciate dog sitters. However, about 4 weeks after the stay, I noticed a lump on my dog's lip. During drop-off, I also noticed that one of the sitter's dogs had several lumps around their mouth and nose. The sitter told me that it's older than 10 years old, and I thought these were just a signs of aging since the dog . Unfortunately, the lumps on her dog look exactly the same as the one my dog has. I took my dog to the vet, and they confirmed that it was a Papilloma virus infection. There is no treatment available, and surgery would be needed if it grows too large.

I reached out to the sitter and informed her of what happened, suspecting that my dog contracted the virus from her dog. I kindly requested that she stop sitting other dogs until her dog is fully recovered to prevent further spread of the virus. However, she replied denying that her dog had any virus and accused me of taking my dog to a park, which I did not do. She said the lumps and skin tags that she has checked regularly that are very common for his breed in aging. When I asked for a picture of her dog's skin, she refused. She showed me medical records of her dog, but they were not related to the skin lump diagnosis. I requested her to share a picture with my vet instead, but she still refused.

My main objective is to prevent this sitter from providing care to other dogs until her dog is no longer contagious. It's concerning that the sitter is not taking responsibility for her dog's condition and lacks integrity. Can anyone suggest how I can prevent this sitter from providing care to other dogs to prevent them from suffering the same as my dog?