See summary of British Citizenship law here
Being born in the UK (or most countries) does not make you a citizen of that country.
You get the nationality of your parents
There are usually provisions for children of people who have Permanent Residence, and have also remaining a country to then apply to get Local Citizenship, but you don't get it automatically.
- 10 year rule.
This is the case for the UK.
She is also under 18, so her parents are legally responsible for her and deciding where she will best looked after. So if they are returning or unable to look after her in the UK.... where would she stay ?
So despite having lived in the UK for a long period of time (on a dependants visa), her passport and her nationality will be Lebanese. She is not being forced to go somewhere, she is basically going home 'returning' (Albeit to one she has not really or perhaps ever seen), It is a nice country though as I remember from when I was there a few years ago. (Nice ski slopes just outside/above Beiruit), Worked with a lot of quite clever people working hard to make it a great place in the future, she has the chance to be in at some of that start. I trust her Arabic and French are pretty good, she will need both.