That is just rubbish. 20-25% end with the child being taken back into care or placed in alternative accommodation or boarding within 10 years.
MOST adoptions that fail do so "unofficially" - so that 25% is less than half the total.
Child moves out at 16 and goes non contact, or similar, or failure after 10 years, which isn't in the statistics, or family struggles on totally broken, or child just quietly moves out and sofa surfs, or similar.
Adoptions have a lower success rate then marriages, which fail at a rate of 1 in 3
Taking on a damaged child is a heroic thing to do, and I am sure that even children of failed adoptions are far better off than they would have been in care all their childhood.
But it is something that is taken on seriously, and quietly, and smoothly, and with expectation of trouble, and likely heart break - that is why the idea of a party seems so out of place to me. But I guess if that is what she wants, go ahead. As long as the child doesn't know anything about it, I suppose why not. My gut reaction was NO!!!!!!!! but maybe I'm wrong and its nice for her