Start by pointing out the practicalities of considering adoption. It would mean roughly two years of paperwork, court procedures and vetting (even though the child is already in her care). It would cost anywhere between £4000 and £9000 in court fees. Once the adoption was completed, she'd lose the fostering payments that she's been getting for the past seven years, cutting her income by around nine or ten thousand pounds per annum. So it's a massive step to take.
In making decisions about adoption, and related issues, courts always put the child's interests first, as hope that everybody would want to do in this situation. If the girl is happy enough with the existing arrangements, what would she actually gain if the adoption was to take place and, equally, what might she lose through it?