A bad plug and wire will not cause an oil leak. It will cause the engine to missfire though, and add a black buildup to the bottom of the corresponding plug. Sense your car was idleing bad, I would think that one of the plugs wasn't sparking. I would tell the guy to check the bottom of the new plugs after a few days. If they look ok on the bottom, then you no its not blowby or valve guide/stem seal leaking.