I would only let them use 310.15(B)(6) if it was sized to the service capacity, not the generator.
If this is a house running on 100a from the utility, then it could use 100a to size the generator feeder.
I am still a little uncomfortable with 310.15(B)(6) when people continue to load up a service after the initial installation until they are actually cruising at close to 80% a good part of the time and tripping the breaker occasionally.
