If he can avoid the safety concerns as far as cave ins and oxygen problems it seems like an incredibly benign hobby compared to a what a lot of people are into. It must be great exercise, and he could end up with some really cool underground space if he does it properly. I wonder how many women out there who's boyfriends spend their time at the bar or at the track or doing some sort of drugs would love it if they were just digging a tunnel at home instead.