What does this mean? Lots of treatments cause harm, or have the potential to cause harm. Sometimes, unintentionally, that harm is worse than the original illness. How are many of the current drugs or treatments in use if drs must do no harm?
FIRST do no harm. Non-maleficence - the principle that all medical professionals must factor in the risk that a treatment could harm the patient more than the condition that you sought to treat. It's a fundamental principle in medical ethics, and taught in all medical schools across the world, as far as I'm aware.
The idea is that you balance risks. There is no physical illness such as cancer here here. You are treating a dysphoria. It should be first principles to ensure that the interventions are necessary and proportionate, especially when they have such potentially cataclysmic effects on a young person's life (both in terms of possible risks and complications, and in terms of the risks that they will regret those interventions).
"First do no harm" is the principle that you ensure you don't leave a patient worse than you found them, if that is reasonably forseeable at the start of the intervention. Obviously, that's never wholly knowable. The only medical interventions to be risk free are placebos. But it's a pretty good guess that treating an autistic young girl, with trauma in her past, and possible anorexia, as though her insistence that she is a boy will fix all her woes... is potentially neither beneficent, nor non-maleficent. Especially if that progresses to care that will render her infertile, may well render her incontinent, and will block any chance that she can simply outgrow the dysphoria and walk away from it in later life, unaffected.
If Keira Bell had been offered proper psychological care and adequate gatekeeping, she would simply have outgrown her dysphoria and moved on and past it". As it is, the effects of treatment will remain with her for life. She's simply asking that proper safeguards be put in place to prevent that happening to thousands of other young people.