Hi Paul
What 8x56 would give her is a larger "exit pupil" than 8x42.
56/8 = 7, so 7mm exit pupil
42/8 = 5.2, so. 5.25mm exit pupil.
Larger pupils do indeed mean a brighter image, but to take advantage of a 7mm exit pupil, she'd need her own pupil to dilate as much, and normally, only young people can reach 7mm (and that's only at night/dawn/dusk). Binoculars with aperture of 56mm (or anything bigger than 44mm) are more suited for astronomy than for birding. For birding, stick to apertures between 30mm and 42mm.
If she is indeed tiny, maybe she'd end up preferring the smaller apertures (smaller than even 42mm). For example: 8x32, 7x35, and 6.5x32 are smaller and quite lighter, and still perfectly suited for birding.
If she needs prescription glasses for far sight (myopia, astigmatism), it's best to get her binos that can be used along with her glasses (instead of taking glasses off and on and off and on again, which is really terrible for birding). This translates to a requirement of "long eye relief", 19mm or longer. This made a huge difference in my case when I upgraded.
If she has unsteady hands, she might benefit from a little lower magnification. 7x and 6.5x make hand shakes less visible/bothersome, and also have the advantage of more depth of field (more of the view is in focus, requiring less focuser operation overall, which means easier to use).
Waterproof and fogproof are a necessity for birding in cold or humid conditions (like forests).
Close focus (the minimum distance at which you can still see stuff) is important for birding. Many people consider 2m to be the largest close focus they can tolerate. For me personally the limit is somewhere between 3m and 4m. Any binocular with close focus larger than 4m should be discarded. (Also, obviously avoid the "autofocus" "fixed focus" "no wheel" binos).
A potential problem is that if her face is narrow, many binoculars won't fit her. The spec is called "Inter Pupillary Distance", and many single hinge roof binos have a minimum IPD that won't fit small faces. You can measure her IPD, but that might spoil the surprise.
It is generally advised that people try before they buy, but if it's a birthday gift you might as well just take the risk for the sake of surprise, just go for a store that makes returns/changes easy if a problem arises.
If she is good with technology, and doesn't own a superzoom "bridge" camera, and you think she might like taking pictures of birds, then a camera could be on the horizon (or be an alternative gift). You can get decent superzoom cameras a "to get started" for <$300 that can go to 40x optical zoom and beyond, and they work well under bright/sunny conditions (see my
gallery here for examples).
Farther in the future, a birding trip to South America could be quite the gift. You can start saving now