Re: undescended testicle
One of the dangers of not seeing a doctor is that his problem might bother him emotionally for years to come. At his age, if his testicle is well and truly undescended, it's probably salvageable and can be surgically lowered into the scrotum. The longer he waits, the more likely the testicle will have to be removed if the issue is dealt with later. In either case (again, if the testicle is truly undescended) both balls have a higher chance of developing cancer because of it, but don't fret! That risk is still quite low. Having testicles that can be felt in the scrotum is important in catching this cancer early.
When I was his age, my left testicle was sort of descended and the other wasn't visible at all. I was too scared to see a doctor. Waiting, hoping, and praying had no effect. When I was 23, they removed my right nut. I'm 31 now, and still wishing I had taken action earlier. Having only one nut makes me sad. Hopefully, your friend can avoid this kind of anguish or regret. Good luck.