The two main things to look at when buying a motorcycle for a woman are seat height and weight. Both feet should be able to touch the ground, but more experienced riders may be able to get away with only one foot on the ground. Also, if the motorcycle tips over, it shouldn't be so heavy that she can't get it back on its wheels.
Tall women or women with long legs won't have as much of a problem finding a motorcycle as shorter women will. A few motorcycles are made specifically for women, such as the Harley-Davidson Sportster XL883L or the Honda Hawk Automatic. Lower seats and thicker-soled boots are also options. Lowering kits do exist, but they aren't recommended, since they adversely affect the handling of the motorcycle and can lower the motorcycle's worth.
Also, the motorcycle shouldn't be too heavy to lift. The best way for someone who isn't very strong to lift a tipped motorcycle is to stand with her back facing the motorcycle and then, using a squat position, lift the motorcycle using the strength of her legs. Trying to lift the motorcycle with her back or arms could result in injury.
Some motorcycle safety classes are tailored especially for female riders.