I am a bartender working long late hours and ALWAYS on my feet carrying big heavy beer cases and liquir bottles, and I have never felt a decline in my energy after turning even just veg. That is not to say I am never tired - obviously we all get tired sometimes. But in terms of low energy - never.
Many vegetarians just go vegetarian without doing any research on it at all. Then the end of up eating vegetarian or vegan junk food, feel like shit, and then go back to meat eating thinking that it is veganism that is taking their energy away. Even if your a vegan - if you eat vegan crap and junk food all the time- then you are obviously not eating healthy and will feel it.
before I can really comment on your friends situation, maybe you can provide a little more detail. How long has been vegetarian? What exactly has she cut out and what does she still eat? And, most importantly, what is a typical breakfast, lunch and dinner to her?
many peole claim they eat healthy, but really, if they keep a food journal, they will realize that they dont eat as healthy as they thought.
Lastly, how long has she been waitressing. I can tell you from experience that if she just started, she will feel very fatigued for the first few weeks until her body gets used to the screwed up hours, always being on your feet, etc. This would happen even if she ate meat. I know when I first started bartending, I was so exhausted all the time and never really felt well rested. I would sleep pretty much all day until I had to start work at night.
So those are some things to consider in trying to figure out why she is so exhausted from her job.
Let us know if you can get a little more info.
And by the way, please encourage her to join this forum. There are vegetarians in this community too.
I am sure that it wont be long before your friend realizes that veganism is a natural next step from veggie. We are here to support that transition in a warm and helpful way.