Remember that it is difficult to leave an abusive relationship. She needs to tell her story in her own time and at her own pace. Listen to her and let her know you care about her and that she is not alone. Take the time to talk with her privately and ask about suspicious bruises or fights that you know about. Validate her feelings.
Abuse is awful, scary, embarrassing and hurtful.
Help her make safety plans. Help her find a safe place to go, or a place to store money and important documents. Let her use your phone if she needs to make calls. Agree to call the police when needed. If necessary, designate a code word that she can use to indicate the need for you to call the police.