Make sure your child understands what it means to be in a loving and supporting relationship.
Once you’re done talking, set a good example in your relationship with your significant other.
Also, while you may not like the person your child is dating, be supportive.
If you have taught them about a proper relationship, then you should trust them to make their own decisions.
You need to keep the lines of communication open and also reiterate to them how they should treat people and expect to be treated in a relationship.