My situation was unusual, so I'm not sure my solutions would generalize. Sciatica in teens is very unusual. The cause, likely, was a series of fairly involved surgeries I had had as a young child combined with overuse. None of the orthopedists I saw were particularly sure about the cause, nor were the physical therapists.

The long term solution was to avoid triggers and increase core strength. The short term solution was NSAIDs and correcting sleep posture (getting an 8-9 hour reprieve from loading the muscles that spasm remains very helpful).

Long-distance running was a trigger for me (particularly running slowly, oddly enough), so I haven't run in basically a decade. I used to run 5 days a week. It was pretty clearly part of my identity.

Years later, I picked up rock climbing (2-3 days a week) and my problems incidentally went away. Whenever I stop climbing for a while, I get periodic minor flare-ups. Fortunately, the short term fixes can nip this in the bud, particularly if I start exercising again.

I don't know the details of their situation, but I would encourage them to seek out a physical therapist. And to change therapists if it doesn't work. It took me a while to find someone that could actually help. I have no idea why the first therapists did not suggest improving sleep posture.