My box has an dryer style vent, at top. The heat from charge controllers, inverter, batteries insures a higher temperature in the cabinet. The higher temperature in cabinet vs. it's environment outside the cabinet causes a convection current out the vent in top.
Additionally, exhausted by a powered fan for 1 minute every hour that evacuates all fumes from building up. The cabinet fan kit I developed will be for sale from the site soon. It is my opinion that the air flow and air evacuation will not allow the gas to damage anything. I myself am putting my stuff all in with the batteries.
I think your friend makes a good point. The reason for the vent, convection style air flow, and powered hourly air evacuation is all counter measures to the gas and it's potential to cause any problems. The amount of gas in 1 hr is so negligable, the fan removing the air hourly makes for a pretty negligable effect of what harm the gas could do.