I'm arguing non-homeowner had zero risk and should have zero equity.

The non-homeowner put zero money down for the purchase, they put none of their credit at risk, they took on no liability for the property, and so far there's no mention of their obligation to pay for upkeep and repairs. Doing those things are the requirements of home ownership while the benefit is the equity. The non-homeowner simply hasn't done the things to be a home owner. If the did, then they'd be a home owner.