The older WDW buses have lifts, but many of the lift buses have been replaced with newer ones have either fold out ramps or ramps that are built into the rear entrance. They are much faster to use than the old style lifts.
She should still enter at the rear of the bus because it is a larger door and will be easier getting the wheelchair on thru the back than trying to get it past other guests in line or who might be sitting in the front seats of the bus. What she can do to speed up the process is tell the driver she doesn't want to sit in the wheelchair while on the bus. If no one is sitting in it, it doesn't need to be tied down as well and could maybe even be folded up and held by another member of your party.
If you haven't already, check out the disABILITIES FAQs thread located near the top of this board. There is one section with bus service FAQs that was written with the help of several WDW bus drivers.
