Originally Posted by
B747-437B
A US Visa can be allowed to expire at any time during the stay. A visa is only relevant when making an application for entry. It grants you no rights whatsoever other than the priviledge of make an application for entry at a POE.
The determining factor in this case is whether the alien remains IN STATUS throughout their physical presence in the USA. Status is granted by the inspector at the POE and is usually annotated on the I-94 form (for most statuses including the most common B1/B2 status). A visa can be still valid while the alien is out of status and status can be still valid while a visa has long expired.
I took the OP to mean that his girlfriend's permission to stay in the US expired on 1 March, but perhaps I was wrong.
Assuming that is what the OP meant, then I wouldn't risk it. It's unlikely that she would be prevented from boarding or incommoded in any way during her departure from the US, but it might cause problems when she next comes to apply for a visa to visit the US, since the date of departure will, as a matter of routine, be sent on to the authorities by the airline. There are no controls by passport/immigration officers when leaving the US, as noted above, but the info is still tracked.
There's also the question of what happens if she is delayed in leaving for some reason — either because of her (e.g. illness, lateness) or the airline, or the weather or whatever. She has no leeway (in fact, less than none).
I would either re-book the flight or extend the visa (if the latter is possible); the former would be a better choice, probably.