She could try to get to her doctors office and get the form and then go to the DMV to get a temp hanging tag, but I doubt that there is enough time this close to your travel date, but it could happen.
There is nothing Disney can issue if that is your question. The only way to get a handicap tag is from the persons local DMV and you have to have a form from your doctor. ( or at least that's the way it works here)
That's how it works all over - a form needs to be filled out by the doctor and brought/sent in to the DMV, who actually issue the permit. (In some places handicapped parking is handled by a different department than DMV, but the process is the same. The state's website should tell you who handles the permits. )
Since today is Friday, not much chance of getting it taken care of, but it might be worth a call the the local DMV to see if there is any possibility of processing it yet today. Many states have the forms online where you can print them out and take them to the doctor and then deliver to your local DMV.
As peemag mentioned, if she does have a wheelchair or
ECV, let the parking lot CMs that direct traffic know you have a wheelchair or ECV but don't have a handicapped permit and they will direct you where to go.
WDW calls their handicapped parking "Medical Parking". Some of the Medical Parking areas have some spots (often not many) with white paint. Those can be used by someone directed to park in the area without a handicapped parking permit.
As was mentioned, spots with blue painted line are designated handicapped parking - whether they have a sign or a handicapped symbol (wheelchair) or not. Guests parking there can get a ticket if they do not display a handicapped parking permit.
Be aware that some of the Medical Parking is quite a hike to the entrance turnstiles. If she has a wheelchair or ECV, not a problem, but if she does not, she may actually have a shorter walk if you are parking in the regular lot and using the parking lot trams.
Another possibility would be drop off. All parks have a drop off point where guests can be dropped off fairly close to the gate (MK is at the Ticket and Transportation Center). I didn't see where you are staying, but if you are staying on site, I would recommend using the bus to get to MK.