Read. Write. Read some more, in as many genres as possible. An English degree will be of use, but is by no means essential. A creative writing program will only be useful if the program is prestigious (ie she will make connections) and covers aspects of publishing also. She should also make sure the programs covers the genres she wants to write in (most focus on lit fiction, which is useful to read for anyone, but useless to learn to write if she wants to write genre fic).
There are no publishing or writing internships available to fifteen/sixteen year-olds in terms of paid work. If you want to work in publishing, you usually start out working for free (and will usually be eighteen). She could request work experience. She could also ask places like Entangled Publishing (whom I work with) if they're looking for another online intern in the Young Adult genre; one of their interns is currently fifteen. This involves reading submissions and offering an opinion; it may or may not help her to improve her writing and will depend on her interest in the genre.
There are people who write publishable novels at her age. Hannah Moskowitz was published at seventeen (I think) and writes popular young adult novels, for example. Kody Kepplinger was 19, I think (possibly 18?) She should aim to submit if she feels she's ready--but the odds, at her age, are very low, and it may not be in her interests if the work is very raw.
Her uncle's opinion is irrelevant if he has no experience of the genre she's writing in, by the way, aside from the fact that he may be able to point out punctuation mistakes. If she has very strong grammar skills, he might offer her work experience in copy editing, though.
This is an industry where it pays to know people. She should make every effort to make connections (online is good for this, but do supervise). Blogging and Twitter are both useful. She should review the books she reads in detail, if she can. All this kind of stuff will come in handy when she turns eighteen and can look for lower jobs in publishing or wants to intern with an agency (although they tend to take university students).
It is worth her keeping in mind that most writers maintain a "day job." Writing does not pay all their bills. Training in another profession will nmost likely be very useful to her.
Most of all, if her current work is a bit rubbish (mine was at fifteen :P) she should not worry. She'll be called a "young" author if she's published before thirty. The very best of luck to her :)