Her waning sight is definitely a factor. For that and many other age-related reasons, I definitely would supervise them at all times. Whenever you hear Cassie give a warning snap, re-direct Boomer and give him something else to do. It's just like regular training: whenever you give a correction, like Cassie's warning snap, you have to give the dog a positive alternative behavior, like giving Boomer a toy or playing fetch with him.