Rover Sitter's dog gets killed by customer dog?

As the customer was going to rebook with the previous owner since there were always good interactions, the sitters roommate calls and says that they didn't want to say anything but the last time the customers dog went to be taken care of with the sitter, the dog killed the owner's dog. The roommates said that their dog was always alpha aggressive with others and didn't really like the dog because of the way his behavior was with trying to hump customer dogs and other customer dogs as well and would become more aggressive. She barely mentions this effect 4 1/2 months later. It doesn't make sense because the conversations and photos show a happy Rover Dog sitter and lots of photos of the dogs happy. When she dropped off the customer's dog back to their owner's home, the dogsitter was still happy and playing with the dogs and asking them for hugs?? But the conversation was just shocking and sad because this should have been reported the day it happened. The timeline doesn't make sense and the roommate said that the Rover sitter doing want to do Rover because she was affected by it and also she had just had knee surgeries and couldn't stop the dogs even at a close range. But the weird thing is that the roommate said that if "the sitter" doesn't want to do Rover any more, that she doesn't mind taking care of the customer's pet again. What should the customer do now that they know thing 4.5 months later? Never in 10 years had that customer's dog ever attacked or killed any dog and it is also a golden retriever and very well behaved and loved by all. The roommate blames herself because she wasn't watching the dogs while the actual sitter had just had surgery and couldn't do anything. They love the customer's dog and don't want it to put down either but the effect lingers with the sitter.