One factor to think about is whether or not he ever wants to run the furnace without shorepower or running the generator.
While I don't boondock much, I've been in a few situations where I wanted some heat but didn't want to run the generator, I've got around 5 or 6 hours of safe furnace fan runtime in the batteries (continuous runtime), which should last at least for a typical night of intermittent generator use.
Though unless he has a high powered furnace fan, that wouldn't need $500 worth of batteries.
If he never needs to run the furnace, then a very small battery would be enough to run the lights when temporarily parked would be fine.
