Lumina Camshaft

I have a 1997 Lumina, 6 cyl, with 154000 miles on it. My 17 year old son has been driving it. About 3 months ago it began running hot and water was in the oil. I paid to have the intake gasket replaced. Two weeks ago it started smoking on the back side of the engine. I took it back to the same mechanic and had the head gasket replaced. Last week the car began running rough and had no power. I drove it back to the shop. I am now told the camshaft is broke. He said this probably happen because the RPMs were reved up to high. Is this possibe or I'm I being suckered?

Mark