Usually a lurker, but wanted to toss my two cents in on a few things here.
1. Absolutely, 100% do not want to discourage anyone. But the things we "know" we want to do in high school are not necessarily the right path. I know for some people it is, but I think having some time in undergrad to explore other options is super important. I "knew" I wanted to be a vet in high school but when I got to college, I started to question it. I shadowed in human pathology, occupational therapy, physical therapy, worked in a lesson barn, tried research and learned a lot about what I did and did not want in a career. Ultimately I went to vet school, but exploring other options is really what made me really know this is what I wanted almost more so than my experiences within veterinary medicine and that type of exploration might not be available if she chooses an accelerated pathway. People change and there are fields out there she may not even be aware of yet as a high school student that she might fall in love with down the road. Becoming a vet should be a marathon, not a sprint.
2. Even if vet med is what she wants, there are so, so many aspects of veterinary medicine and rushing head first might wind up costing opportunities to delve into different areas. I "knew" I wanted to be an equine vet in high school, through college, and even when I graduated and decided to work for a few years instead of applying to vet school. Fast forward, I took a job at a lab and I fell in love with primates and I am most likely going to wind up in lab animal medicine. Never in a hundred million years would high school me have even considered being involved in animal research or working with primates. Again, interests and perspectives change and it might be worth taking the extra time to grow up and get to know more about what she wants...and also to just enjoy being young! Remember, marathon not a sprint! 🙂
3. Regarding the working vs. shadowing thing. Considering she's in high school, a paid job would likely be hard to come by. But also, shadowing is much lower commitment and gives more freedom/flexibility to say "hey, this isn't working for me" and go try something else be it a different clinic, a different aspect of vet med, or a different field altogether. When you start working you become a little more "stuck". So I absolutely would advocate for at least starting with shadowing. Another thing is when you start working suddenly you're cleaning exam rooms, checking people out, restraining patients, putting in bloodwork forms, answering phones etc. All 100% great experience...but it does wind up limiting your time to talk to the doctors and spend time getting their perspective about the pros and cons of the field. When you're shadowing, people aren't relying on you to be somewhere or do something in quite the same way.
Anyways...I know this stuff doesn't necessarily answer the question about accelerated program but I hope it provides some food for thought regarding whether or not an accelerated path is the way to go or not. Best of luck to your sister whatever she decides! 🙂 I just really tend to advocate for slowing down, trying everything, and getting some more life experience before committing to the long and costly path that is vet school.