I recently did a 90 day reporting online it was submitted and approved within hours, go tomorrow or within the next 7 days into your local immigration office and do it(cancel the pending application)
just a recommendation as it will save him time/money(unless he wants to spend more money then he needs to) doing a border run(bounce) may attract a fee when getting a Visa at the border/land crossing plus travel cost from his location return(unless he likes travelling by car)
As a general rule, you show up on the day that is stamped in your passport, unless there is a delay in your paperwork on their end, then they will give futher instructions. SHOW UP, they do have the power to cancel your application, and give you 7 days to leave the country.