The form your friend filled out only states "
proposed date of entry" and I think the possession of a visa close to that date will do it. Rather than deal with a new form, new fees and yet another passport picture, I would be willing to risk two hours spent with occasionally scowling people in uniform.
Mind you, I am not the ultimate authority on this, but I just returned from Vietnam using an American passport. As to the airline, it's as
jrhone says, and perhaps your friend is connecting to a later flight than he actually has anyway.
This relates to the situation that one of my friends is in:
He has a visa to visit Vietnam, but the dates are wrong and he will be arriving in the country at 10pm the night before his visa is valid. He is willing to just wait around until midnight before going through immigration. The question is will the airline let him even get on the plane?
He is going to get this issue resolved before leaving in over a month just to be safe, but it would have been terrible if he hadn't noticed until just before.