I get that as well and have been told to be careful with the juicing, but I figure the cleaning is so good for me and I usually lose weight on it that I go ahead and do it. She may want to be careful. But as far as getting jittery I don't get that when I am doing the juice as I get it through out the morning. Remember you can have as much as you need and I find a steady stream was good for me. So that helps stop the low blood sugar thing. But I have also read that she can have a few nuts to chew on with it. So that could be the protein you are looking for but I do think that that would slow down the cleansing. check on here for more experienced juicers. There is a thread somewhere.
You could always start with the water cure. I think that there is a thread on here for that as well.