The book you're looking for may be "A Day at the Zoo", by Marion Conger, pictures by Tibor Gergely, LGB #88, copyright 1950. I checked my copy and there is a two-page section about the polar bears, besides other zoo animals. You may be able to find a copy through online bookstore searches; hope you find what you're looking for for your friend!
A friend of mine told me about a book she loved as a child . She said it was about a polar bear and the zoo ... back around 1956. Does anybody have a thought on what the title of that book might be ? I'd like to buy it for her birthday coming up in Aug.