Helping a friend understand

I had a talk with my friend, I would say he leans more towards the agnostic side of things, he values many Christian teachings, but the one thing holding him back from believing is the fact that if God were to be All-Powerful, All-Loving, and All-Knowing, why would He allow a kid with cancer to struggle? I tried my best to explain it to him, but I’m wondering if anyone else has had a similar scenario and if so, anything I can say to help him understand? All help is appreciated.