I would definitely recommend having a first "real job." There are things she can learn at a real job that will prepare her for whatever she wants to do next. I probably would be twice as good if I had worked two different jobs, and three times as good if I had worked three. Every experience gives you more knowledge, which then guides you as you make decisions about how you want to act, what you want to be, how you want to be seen, etc. It will also teach her about about how people write emails, what makes a good manager vs a bad one, what makes a good co-worker vs a bad one, how annoying a rambling 2-hour conference call is, what makes a good client vs a bad one, etc.
There are also the itty-bitty things that I didn't even realize were things. I hired two college kids to help me part time at my old job and after their first "here's the design I did" email, I realized I had to start with the basics. I didn't even realize my first manager had taught me these things, but he had -- Here is how you start an email. Here is what the email contains. Here is what your signature should look like. No, never use your personal email address for work-related business. Here is how to determine what order email addresses go in. Here is how to CC, when to BCC, how badly BCCing can go. When to call vs email, how to set up a conference call, man the list goes on and on.
It will also give her the chance to potentially be in the other person's shoes in the future. One thing I've learned since managing my own people is that managers have a thousand different things in their head, only one is your situation. Just tell me what I absolutely need to know and move on. I really don't care why something is happening...or how it happened...or whatever. I'm sure some of the emails I wrote about a bug or something would make me cringe today.
I would also tell your niece to opt for an environment that will teach her the most and give her the most opportunities to learn, rather than just looking at the money or company-name power. You obviously have to weigh the pros and cons, but a smaller company where you are wearing multiple hats will give you a lot more opportunities to step up to the plate, interact with other departments, interact with high level management, and be that much more well-rounded. There are benefits to working in a very organized company and learning a strict process, but the reality is, you will learn more about the process and internal workings of the company than you will new skills.
I think one of the most valuable things about my old job was literally where I happened to sit. I sat upstairs with the engineers because I was too young to get a real office and the cubes were full. However, I worked mostly with the marketing department and I also reported directly to the president or CEO for projects for clients or high-level internal projects. This gave me a very unique perspective on both sides of the fence. I learned the information that marketers wanted to know as well as the engineers. A lot of times I acted as "translator"/ "e-mail rewriter" between the two departments because I could speak both languages. Later, I was able to streamline some items in our process that were pissing off the engineers and the marketers, but know one knew (1) that it was really an issue worth fixing or (2) that they both felt the same hatred for it.
Okay - last thing. Jobs also give you a network of people. Companies with high turnover rates are more valuable to you (but more dangerous) because the people you meet and make friends with today will be at a different company next year and can provide you with an in or a helping hand when you need it. You never know when the guy sitting at the cubicle next to you, that annoying sales rep, the IT guys* wife, or whoever can help you get to where you want to be. Your reputation is gold in certain industries so don't talk shit, never say no, and listen more than you talk. Save for maybe one or two truly horrid people, I would move mountains if any of my old co-workers needed assistance with anything. I even liked one of the sale's reps stupid facebook page the other day!
* Always make best friends with the IT guy / gal / team. Bring them cookies and stop in to talk about whatever they are into. (1) The lock on their door and the fact that they have more free-time than they would like anyone to know means they're room is a great place to hide when you just can't take it anymore, (2) They usually have a secret room which is glorious for hangover naps and (3) they will answer your call gladly when you need them and (4) they are always the first to know everything and they usually know all the company's dirty little secrets - this comes in handy when you need advise or can't figure something / someone out.