If a consumption of a small amount of juice in water would support the idea to continue fasting (when a faster considers giving up fasting due to, for example boredom or a gnawing feeling of deprivation), I think that ketosis would not be stopped at once; However, a repeated consumption of any juices would probably reduce/stop ketosis and start starvation.
I do not use cheating on water fast, because I am serious about my desire to have my full health back and do not want to risk starvation. I know, water fast, especially a long one, is a hard lesson to learn and requires a very strong will.