I wear an Acerbis women's chest protector, and it works great. I bought mine a couple of years ago, and Acerbis was the only brand I could find that made a "female" version. For the rest of my gear (jersey, pants, boots, gloves, etc.), I wear men's items in the smaller sizes. You should probably have your wife try on her gear before you buy it, if possible, to ensure a good fit (for both comfort and safety). Good luck!
It's been said already, but... have her try it on. Women are all cut very different so what might work for me won't work for the next. I like the way the Acerbis Zoom II (youth) fits me. It's not bulky through the chest and back and it protects my shoulders from being ground to the bone when I hit the dirt. Then again, I'm 'petite' :silly: enough to get away with it. You'll want to find her something that she feels protected and comfortable in. It's worth it to take your time in finding her the right gear.