If he has been diagnosed (as you put it), then the NHS pathway for under 18s (and idc over 18s) will have been explained.
If she does not wish to follow that pathway, then she will need to go privately. But I would expect anyone prepared to do this will follow very similar protocols to NHS.
If she rejects counselling, it's unlikely she'll be able to secure hormone, let alone surgical, procedures in UK from either sector.