The police routinely take license plate numbers just in case something happens later and they need to remember that number.
Yes, you can drive "her" car, whatever that means.
I disagree with New York attorney Theodore Robinson, who appears not to have understood the question.
You state that the police officer took the license plate itself (not just the number). I have no idea why he/she did this, but in any event, you may not drive a car without the proper plates.
The police can confiscate plates if they are fraudulent, are not for that vehicle, or are evidence in a case (such as a false registration sticker situation).
No, you can't drive without 2 plates on the car. Both a front and rear plate are required under Vehicle Code section 5200.
I am in 100% agreement with Mr. Kaman - please ignore comments from attorneys not licensed to practice in the jurisdiction where the question is posed. They're usually guessing. Worse, they're often wrong (as is the first answer...).