The guy is a kook who showed a total lack of regard for the safety of his helper. A 2nd degree murder charge is appropriate.
The question about “total lack of regard” is something that looks very much up in the air at worst. If two competent adults carry on a mutually agreed and understood activity on property that one or the other owns, and it goes wrong and one dies, it doesn’t sound appropriate to convict of criminal charges. There was no malice; only mutually agreed foolishness. Call it the “hold our beers and watch this” principle. Civil liability could be different, as in his family suing.
And... though this itself would have been a violation of civil law, it wouldn’t be a criminal activity. Had no accident occurred and the work later been discovered, the homeowner would have been dinged with a civil fine and an order to close the unpermitted tunnels. It’s not even like a joint venture to go rob a bank.