And you may be right! I don't think there's a standard, although the few I've measured have been around 11-1/2" in depth.

12" is the standard depth. Keep in mind that the back wall of the cabinet will eat into that depth. And if doing inset cabinets, you will eat into it significantly more.

If she picks a larger plate and has standard upper cabinets, perhaps she can do the dish drawer like so many of us favor - many of us use a deep drawer to put our plates, etc.

I was referring to the inside measurement, which is what will determine whether or not the dishes fit. I just measured mine, and it's 11".