The relief valve could be defective. It may need changing. The tank could also be over heating and that is why the relief valve is releasing. I would check the settings to make sure it is not over heating the water on the domestic side. He should call tech support for more info on the settings, most manufactures have a tech support line.
As for piping it to a drain, sure this could be done but this is usually not done because if the relief valve is a problem or it is relieving for some reason, when piped to a drain you will not know it has opened and you will be wasting water or missing a problem that should be attended to. Like over heating the tank.
Hope this helps