My male friend is complaining of nerve pain in testicle what could it be?

Hernia? This could be a number of things, including a hernia, a pinched nerve in the back, or a problem in the testicle itself. A family doctor or a general surgeon would be the person to see and be examined by to make sure.
Make sure not twiste. If it is twisted this is a medical emergency - so make sure it is not this. Sometimes kidney stones can reflex to this area - testicular pain could be due to trauma, testicular torsion (a medical emergency), infection, hernia, inflammation, tumors, or kidney stones. For kidney stones I have used super phosphazyme by biotics research take 15 drops + agua mag chlor both- biotics dissolves stones.