You have some oil control issue, compression or weak ignition associated with that one cylinder. That left plug is not acceptable and should look like the other. I have an old craftsman twin that did that, but not as bad, it was a bad magneto coil.
A little soot doesn't confirm that he has an oil consumption issue. He should just run the machine under load for a while then take a look at the plugs. If the engine isn't smoking blue under load, not likely to be an oil consumption problem.