A friend of mine bought a 2000 Camaro Z28 this past week. His gauge inside is reading about 18 volts. He checked it out and it really does charge at 18 volts. What are the dangers of overcharging a system? and what could he try to fix the problem?

 

Well, 18 volts isn't horrible, ideal is something like 14.2 volts I think.
Aside from popping fuses left and right, possibly burning up batteries quicker, I suppose there's a chance it could damage his ECU if it ever peaked.