The Draft for a handfired should be around .05"-.07"
It sounds like he has plenty of draft when the fire is hot, but once the temps drop off, his draft drops, this reduces the air through the fire, further reducing it's temperature,, and soon the fire dies..
If his house is really tight, the chimney will have difficulty pulling a draft from the sealed house.. He may need to provide a dedicated outside air source to the stove area..
The manometer will tell the tale...