How? I mean the way every drug for example is tested for safety, they give it to people (healthy volunteers) in escalating doses, and watch them for adverse effects.
Rarely, it happens that a drug that was expected to be safe based on animal testing turns out to be dangerous in humans, and healthy volunteers die. It’s a tragedy, it happens rarely, but it happens, and safety trials are still the best way to start the process of clinical trials for new drugs. For some treatments, such as gene therapy, it’s not ethical to ask a volunteer to just try it to see if it’s safe. So they look for sick people, who are consenting to risking a new treatment with unproven safety, because their other choice is to not be treated.
This is how it’s always done. I wonder what you mean when you say the treatment should be tested for safety before giving it to patients.

Edit: this was a personalized experimental treatment developed specifically for this girl, on the parents’ initiative. The clinical team did test for antibodies against the virus before injecting the AAV into the csf. Lots of shady and wrong things on the research side, I’m not protecting them, just disagreeing with the previous comment that this tragedy would have been prevented by a simple test.