Cutting header on load bearing wall
We have a garage door that needs to be replaced. The joists on the garage run perpendicular to the wall with the door, which supports half the attic if not more. The door is a normal single pre-hung door. My father went out to buy a door for the RO, which is 81-1/4", but the jambs on the new door measure 81-3/4". He is insistent that cutting the header on this door to make it fit will work. I'm worried more about the header collapsing without any support. The header is 2 - 2x10's. He says cutting an inch out of it won't have any effect on it but I compare it to stretching cloth tight and cutting a slit making the cloth rip apart. Please advise.