One of the things you can do is help her to build friendships at school. Invite over, 1 child at a time, some kids from her class for playdates.
Help her to discover things that she's interested in. Kids that are passionate about a topic (for me it was horses, horses, horses), I could talk about it to anyone who would listen. My parents supported by interest and encouraged me and becoming proficient in something helped me to gain confidence. And, no, we didn't life on a family; my parents scraped money together for riding lessons, and I lived for those lessons, and took them for many years in grade school and am still passionate about them.
One warning: avoid calling her shy. She can feel shy, and as we all know feelings can change. But you say she "is shy" you are putting her into a box where she "is" that thing, 24/7. So, when people talk about her, say she is an introvert (read up on introverts vs. extroverts - it's just a personality type and both are need to have a balanced society), and if people call her shy, tell them, yes, sometimes she feels shy, but no one is shy all the time!