I read JW from about 8-11. I went on to read English Lit at a college of the University of London so I don't think a bit of JW held me back at all. I was - and still am- perfectly happy reading classics, Booker contenders and medieval passion plays in the original alongside chick lit or crime novels.
Quite honestly you need to leave her alone. Going on about "unsuitability" just breeds curiosity and trying to get her to read what you want her to read is more likely to put her off, particularly if she already isn't that interested. Summer holidays are also good for recharging - she may not be in the place to tackle something worthy right now. Don't put her off reading. Just tell her it's great that she does it.
I also grew up in a naice family in a naice area and attended naice schools. A lot of the issues in the JW books were also my first real exposure to people living different, more difficult lives in different circumstances or seeing the direct impact of things like divorce. They made me more empathetic. I remember reading Vicky Angel being the first time I understood grief as an all pervading, lasting concept.
I never read some of her first novels which were seemingly aimed at older teens, although I probably would have been allowed. My mother bought JW books for me and never said a word against them- I think she thought they widened my scope. That said, my parents were very open and there was very little censorship in our house. The only thing banned was Jackie Collins, my mum thought she was crude. At 12 I was reading Meg Cabot (Princess Diaries) and Louise Rennison, still collecting Lemony Snicket, all the Anne of Greengables and Little Women and various Anthony Horowitz books but also getting into Patricia Cornwell, Jilly Cooper, Dodie Smith, Maeve Binchy, Daphne du Maurier, Steinbeck, Laurie Lee, Margaret Drabble, H E Bates and others. One of my favourite books at that was a collection of stories of people who had encountered Anne Frank whilst in concentration camps.
When it came to YA novels though, I much preferred real life, slightly gritty or girls issues ones or straight historical fiction. Joan Lingard was a favourite. A lot of the YA stuff around at the time was fantasy and I've no interest in it so I did go looking for adult books rather than plod through Garth Nix to please my absolute cow of a school librarian who viewed a dislike of Dianne Wynne Jones or the Hobbit and similar as a character failing.
Also like a huge chunks of MN I make no apologies for a life long affection for the Chalet School.