A boa of 6 or 7 feet could very likely kill and eat a cat. The more important question, I think, is why you would be concerned about this in the first place. There is no reason I can think of why any snake and any cat would ever be in such close and unsupervised proximity to each other that might result in this situation ever arising. The cat could also harm the snake when it was too young to protect itself. Cats are predators at heart.
So yes it's a possibility and if it occurred it would be the owner's fault for not keeping the snake and cat apart and safe from each other.