While vitamin C is not a neurological stimulant (like caffeine is), some people want to believe it is because it makes you feel so much better when you get enough of it. I actually saw a post on a discussion forum once where a women claimed vitamin C was a stimulant because she felt more awake when she stopped drinking coffee and replaced it with orange juice. However, caffeine results in a "crash." Anyone that stops their intake of caffeine long enough to get past withdrawl effects, starts to feel better, so she is (along with many others) mistaken about the actual cause of her energy uptick. Believe it or not, Caffeine (and other stimulents) make you feel more tired when you are not on a caffeine high. Oragne juice will not leave anyone with this after-effect because it is involved in the formation of "real" energy.
Here is what Citric Acid (which is also contained in Orange Juice) really does with regard to energy levels:
According to an article posted by Livestrong one study found Citric Acid reduced the production of a specific salivary component that is considered a stress marker - in other words it guards against the effects of stress (or at least the one). Anything that reduces stress helps you feel more energized. As an anti-oxident, It has being found to have impacts on many of these "stress" markers based on what I remember from biochem in college.
It is a major component in producing chemicals needed in metabolic activities (The Citric Acid Cycle, for example) that help increase energy production at the cellular level (stimulants do not increase energy production, they are a "fake" in that they make the nervous system more active they are not actually involved in creating honest energy in your body. Science Daily gives a brief definition and some examples of actual stimulants).
Here is why people sometimes might believe Vitamin C increases energy levels:
Since Vitamin C helps in regeneration of other important anti-oxidents in the body, it is probably even more important in this role than other well-known anti-oxidents such as Vitamin E. This also can give a person a sense of having more energy.
Additionally, vitamin C helps guard against immune system deficiencies and acts as an anti-oxident. Again, this helps reduce energy drains when you don't have enough vitamin C - if she had a cough she may have more need for immune system production than usual and a little extra vitamin C in the morning might not be a bad idea.
So, while neither Vitamin C, nor Citric acid are stimulants, it does impact energy levels. When combining Vitamin C with the sugars and other carbohydrates present in a cup of juice (as explained by Beofett), it is likely to cause a certain amount of wakefulness in the drinker. Next time she wakes up in the middle of the night, I'd stick with a glass of water (it is better for her teeth that way too).
For those wondering about my statements on Caffeine: