I fully understand & empathize with your Fatherly instincts that tell you he is a poor & possibly dangerous choice. Ask her open-ended questions about him & their relationship that are non-judgmental, shaming or blaming. Don't put her in a position of having to choose between her family & him. You have instilled your beliefs and your values in your daughter. You need to believe that you have done your job in raising her to make sensible decisions about how and whom to select as a boyfriend.