Make her understand her situation by recounting behaviors and events you have witnessed. Tell her you are concerned about her safety and implore her to talk to you about what is happening. Listen to her and let her know you believe her—after all, her abuser has very likely told her that no one will never, ever believe her claims of abuse.
Still, a victim of domestic violence can be in denial. She may not accept the reality of her situation until she is ready to. In some cases, the best you can do is to continue to be her friend in a nonjudgmental way, until she eventually asks for help. It may take time. Let her know that whenever she is ready, you can offer her a safe place to stay until she gets back on her feet.