Tommy's dad didn't steal tank of 6N helium, but it did rolled off the same facility using the same process, main impurity being air (considering it's a rather minor use, maybe balloon gas is just what is left after cleaning or purging empty helium tanks of higher grade. so it's maybe not a massive loss. recycling helium within cryogenics and MRI would provide more benefit)
also wtf is "grade", 6N means "six nines" means 99.9999%. (americans will use anything but metric units) liquid helium freezes everything else out so it's 5N without any special extra purification, or at least that's my impression from looking up spec sheets of helium from facility that i know uses cryogenic purification for it