A child who is adopted will have invariably have experienced some form of trauma, a number of severed attachments.
The reason the outcomes for children who are adopted are so improved from children who grow up in various foster placements, for example, is that they move into a stable, loving family. They become part of that family. They are enabled to gentle bond with one or two primary attachments figures, their parents.
For children with issues around attachment, breaks in care from that care giver, care giver leaving, going to hospital, even staying with grandparents can feel very challenging, because they have experienced broken attachments in the early stages of life.
The reason as I see it, that boarding school is not appropriate for the vast, vast majority of adopted children, is that you are then essentially moving them from the family you are working so hard to establish for them, into yet another new scenario. They are cared for by multiple staff members, who are paid to care for them and are liable to take maternity leave/get a new job and have perhaps 20 other children to care for.
I think that whilst boarding school may have worked for your friends child, that is probably more from luck than judgement. In my opinion, boarding would be the worst possible thing you could do.
*apologies for use of 'care giver' - I wanted an all encompassing term for all the varieties of families who adopt!