Your mechanic probably did a running diagnostic on your engine. That would tell him if a plug or 2 was miss firing. He would have had to remove the plug to tell if it was fouled or not. As for your Cat., if it is clogged it should effect the exhaust coming g from Both mufflers not just one. I think when your friend drilled your Cat., he loosened a clog in that muffler. There was a thread here where a squirrel made a nest in one members muffler. Clogging it up the first time he started the car in the spring.
I would say, change your plugs and check your wires first. It doesn't make sense to me that a bad Cat. would only effect one muffler or one plug.

 

Changing plugs today..also having the cat removed and a straight pipe welded on.Thanks for all your help

 

Changing plugs today..also having the cat removed and a straight pipe welded on.Thanks for all your help

 

Won't removing the cat cause the engine light to come on? I would think the after cat oxygen sensor will notice the exhaust isn't as clean as it should be, and then the computer is going to turn on the engine light.