She was 13.
13!
If a 13 year old wants a tattoo, you say no.
If a 13 year old wants to drive a car, you say no.
If a 13 year old wants to buy cigarettes, you say no.
If a 13 year old wants IVF, you say no.
If a 13 year old wants to leave school and get a job/join the circus, you say no.
If a 13 year old wants to buy a whisky in a pub, you say no.
It doesn't matter how determined they are. The law is that they are not old enough to make that decision.
Several of these things have much less serious consequences than irreversible major surgery. So you say no. The next surgeon, and the next, and the next should say no.
THIS. ABSOLUTELY THIS. How did we end up with a f*cked-up ideology that sanctions underage children to be subjected to FGM because they believe they want it?
IMHO treatment (whether surgery, hormones or puberty blockers) should be given to under-18s. No matter how much they ask for it. End of. The teenage years are tumultous and no child should be allowed to do irreversable medical damage to themselves. If, at 18, they still feel they are trans after going through puberty (as those with genuine gender dysphoria would) then by all means they can decide to crack on with treatment.