My husband is scarred for life by being sent away from a happy expat posting thousands of miles away (I think you are HK so you will understand a happy ESF primary on Kowloon side) to a British prep at 7, to the extent he has had counselling. The schools are much much better at handling homesickness now but boarding doesn't suit every child and he point blank refused to send his own children to boarding schools and we moved back from a very happy posting rather than do it. I have been UK guardian to a few expat / overseas pupils now and the vast majority wanted to come and settled well. I did however support one 10 year old who was sent and he was truly heartbroken, unhappy and sad and has been badly affected by the experience, as much his parents fault as the schools, but still sad.
As well as involving the school's pastoral support, does your friend's DC have a UK guardian who knows her / understands the cultural difference, who can give a taste of home, food etc. and some space to think and talk away from school.
Is there an underlying issue? Is she being excluded? Perhaps not blatant bullying but Brit girls can be very manipulative and cliquey, with it being worse in particular years (year 9 is the nadir) cohorts and schools. We have learnt the hard way that if that is happening then a move to another school might make all the difference, my DD is an excile from a particularly gruesome year at the school my older DD thrived in and is now transformed at another school, confidant, happy, lots of friends and realising it was the dysfunctional year not her.
Are there other girls from her culture / country in other years who can support her?