Look at Police Supply places, they do make special amor for women. Designed for their body types/frames. It is more expensive ( sexist pricing due to limited numbers) but a good fit is important here. Fitting will be like a bra fitting. Most males might not get that explanation. The best option is actually try it on and/or have it fitted by professionals. If measurements are needed try having them done by a real seamstress/wardrobe person like who might work at theatrical costume shop aka not Party City..... Their size measurement sheets will help the makers cut and sew exactly to her size, if you have to go custom.
As far as if for just if things go way south, then the newer military gear that is designed for females is starting to be available. Plus more coverage options for sides, shoulders, groin... Although she should expect to wear double sport bras for extra restraint for placement since plate cuts will be issue for shifting and binding parts of the body. Although this isn't as big of deal due to her size verses more extremely large chested women. That being in the C/D cup and up range especially.
This is an out of stock link/option but might help you on your search
http://www.keepshooting.com/female-body-armor-iiia.html You may need to do a layer of form fitting body armor then a plate carrier and molle chest rig for any gear on top. The soft armor would help produce a flatter and/or sloping surface for the plates to back up against. It would suck buying twice to build the system but compare it to the cost of a wedding dress........