You have probably shared a lot of personal stuff between you, as friends, so it probably seems weird for your friend to have a “secret,” but some things take longer to process and are often figured out alone until the person herself is more comfortable with the idea. Try not to take it personally if she doesn’t open up right away. Just aim to be patient and supportive.
Sometimes people who are experiencing a shift in their sexual attractions hint around about their sexuality or try to gauge the opinions of close friends by bringing up other people who they think to be bisexual or gay. Use these types of instances to let your friend know that you are not judgmental about such things. Eventually, she will probably either come out to you or present an opening for you to ask about what’s going on with her. Then, there’s your chance.
All the best!