I would try to have discussions with your daughter that do not put her on the defensive about being with this young man. Ask her open-ended questions about him and their relationship that are non-judgmental, shaming or blaming. Don't put her in a position of having to choose between her family and 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. If you do sense that this young man is inflicting any physical harm or mental anguish on your daughter, you need to intervene. If not, it might help if you extended a more welcoming attitude toward this boy and made more of an effort to find out what your daughter sees in him. I'm sure that she told him long ago that you all dislike him, so chances are he feels uncomfortable in your presence. Remember, when you talk negatively about her boyfriend, you are indirectly talking negatively about your daughter.
I know that it's tough not to be alarmed when you believe that your children have made seriously bad choices in friends, girlfriends, or boyfriends. I would focus on maintaining a loving, supportive relationship with her and show genuine, interest in the other aspects of her life that do not directly include her boyfriend. You are free to be honest about why you do not like her boyfriend, but it is unwise and unfair to condemn, embarrass, or otherwise make her feel badly because she is with him.