That child is receiving treatment in the UK.
She and her family are covered in Commonwealth and EU countries. (What this means is that when they are visiting these countries they will receive the same health care as any citizen of the country where they are staying.) The NHS does not cover UK citizens when they are in the US because there is no reciprocal agreement between the US and the UK.
At least this child gets the care she needs, and the treatment doesn't bankrupt her family.
I'm an American who could have bought insurance for the kids and me, for several thousand dollars a month, with huge copays and deductables if we had stayed in the US. Of course, every policy excluded treatment for my toddler daughter's Perthes Disease. Instead, I followed my late husband's wishes and moved our family to his home in Scotland where we receive excellent care from the NHS.
Despite their high costs the American policies I looked at didn't cover travel either.