Although the manufacturer's have made strides in making ladies pants and jerseys, as far as I know there are no women-specific boots out there.
Your best bet is to take her down to your local shop, and have her try on a few pairs to see which ones fit best. As a general rule, boots will be 1.5 to 2 sizes smaller than what your wife wears in women's shoes. (i.e., if she wears a size 8 shoe, try on boots in the 6 to 7 range. But remember, manufacturer's sizing will vary somewhat). She should be wearing thicker, MX style socks when she tries the boot on. Boot should be snug, but NOT tight. Boots should not pinch in any areas when flexing at the ankle. Have a set of knee pads with you as well. Boot calf should fit, even with knee pad on.
The best fitting pair of boots I ever bought was my AXO RC-3s. I have a set of Alpinestar Tech 6's now, but they are a bit stiffer than my AXOs. I tried on Alpinestar M4s at the same time as the Tech 6, but didn't think they fit as well, and they pinched a bit in the ankle area. Tech 8s were just way too stiff for me, and not worth the extra $.
Good luck!