Can bad valve cause CAM damage?

I bought this vehicle from my mechanic.350 chevy engine.I have only driven 70 miles under 40 mph in town.I noticed a hesitation in it.I took it to him and he had one of his employees look at it.I told him the carb was leaking and choke not working and would not open all 4 barrels either.I went to pick it up and the guy whom worked on it said there is nothing wrong and that he had started it and did a "power stance"for 5 minutes!He did no other checks! When I left ther I noticed now there is a Knock not just a miss?I changed plugs and replaced Carb. When I started it bang was still there. Did not drive it. Took it back to him and he said it had a bad valve.2 hours later he said the cam was damaged and motor is no good?Not sure how bad valve damaged cam? But I do know with a miss you dont want to rev engine up to do a power stance. Did he cause this damage by doing this causing more damage?