Using the “on” position rather than “acc” on an '03 Accord is not a problem. It does use more current, but for short periods should not run down the battery.
In older cars some electrical ignition parts could fail as a result of this practice. I burned out the ignition module on an '85 Ford LTD midszie wagon by leaving the ignition on by mistake.
Is it a good practice to do this, no. Is it harming his car, no. Did it cause the battery on his car to fail quicker than yours?, not likely.
If you want to keep arguing about it by all means go ahead.