If your vanpool-mate is reporting 100% of his mileage as LLC-related, than he's committing tax fraud. The tax forms for reporting business expenses require you to list miles driven for business versus total miles driven, and require you to have detailed logs for each trip claimed. If he gets audited, he's probably screwed. If you feel justified, reporting stuff like that to the IRS is anonymous, and gets you a bounty based on how much additional tax they collect.

We have some rental properties, and some years we report zero miles because we don't need to go work on them. When we do have major renovations that we plan to work on, we log every trip on paper.

Educating anyone benefits everyone.