You say it's a class and you didn't choose it. What kind of class? Is it a diabetes education class?
If you're diabetic or insulin resistant, then juice tends to be a poor choice because it's just the sugary part of the fruit or veg, without the fiber to slow the sugar down. (In the olden days, when there were children with type 1 in school, they taught the rest of us to give them juice if they seemed to be in insulin shock, because they thought juice would enter the bloodstream even faster than candy or a packet of sugar.) Even if you're not diabetic, for some people juice can cause a sugar rush that leaves them hungrier than if they hadn't eaten anything.
My real concern, though, is why you would listen to an herbalist when you've got access to a dietitian? A dietitian is a person who has a degree and has passed a licensing exam, who honestly knows what s/he's doing. Anybody can call herself an herbalist, on the other hand. It's like listening to somebody with a sign in front of their house saying "toothologist" and ignoring your dentist. The herbalist might have some interesting ideas, but s/he's really no more qualified to advise you on diet than your manicurist is. You might halfway listen to someone like that if you didn't have any better source of information, but when you have a qualified professional, their advice is a lot more likely to be on track.
When you juice you lose a lot of the satiation factor in vegetables. The fiber is not incorporated which is the reason vegetables satisfy. That's why it is suggested that juice portions be limited - I believe it is to 6 ounces a day, but someone can correct me on that. But - there is nothing magical about juice that you can't get from the regular vegetable servings. If you can drink it, fit it in your calorie range and be satisfied, that's fine, but it's not magic.
I don't think the calories are different, and I doubt you will be off by so many calories, especially if it is one drink a day.
I would try this before a workout to give you energy. By making it a liquid, you will make it easier for the body to process, and you will use it quickly. You'll burn it off during workout, and can have a small post-workout snack with some protein.
I think her concern is if you were drinking 4 shakes a day, and were off by 50 each based on size of the fruit/veg. This could be 200-300 calories a day, which might make a HUGE difference. You should have asked all these questions of her, and not left till you agreed on a plan. Better dialogue with you dietician is necessary. Remember, you went to her because she is a professional. You wouldn't argue with your cardiologist about heart issues.
Usually, a dietician will try to keep with a theme.. juicing sporadically, and recommend how often, and discuss how to do it effectively. Sometimes it is just dangerous, and the dietician has to give you a warning ( I am diabetic, juicing would make my blood sugar skyrocket ), and you should listen to it, but she is probably just wanting you to think more on the topic, and how you will apply it.
I have added juicing as part of a healthy diet. I think dieticians get worried that people will juice more fruits than veggies and that they will use it as their only veggie source. Given that there is less fiber in the juice, I have found that it doesn't fill me up as well as whole vegetables, but it tastes good and makes a nice supplement. Once I cut up the veggies I measure then and add them to my tracker before juicing. Also for sweet I try to use apples and carrots or blueberries (but in moderation). Jicama, cucumbers and tomatoes also help a lot. I find it works well to use this as a supplement to a yogurt or protein bar for breakfast, but I still eat whole vegetables at lunch and dinner. As for the carrots, for my money and taste, I buy 2-5 lb bags, peel and cut them up myself.