You may want to have your vet check your cat out to see if there is any medical problem triggering the attacks. If the cat is not feeling well it may do a first strike just in case the dog might attack it.
Thyroid problems for example can trigger aggression.
If its ruled to be a behavioral problem you may find the product Feliway - the cat appeasing pheromone - will help you. You can find it at pet supply stores and online. Perhaps giving the dog some scenting of that will end the cat's attacks on her former friend.
Cats do play very roughly sometimes and if they don't have another cat to play cat aggression games sometimes they will choose another to do it to - including people or other pets so that might be a part of it too.
But first check the health as a sudden behavioral change in a pet often is due to an illness.
Hope this helps you!
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