How To Deal With Relationship Issues?

my daughter is 20 and crazy in love with her first boyfriend. He is controlling, has anger issues and threatens her to end their relationship if I interfere. She begged him and cried to him to please don t leave her because she can t leave without him. He accepted but warmed that he will not put up with me. What can I do or say?, do I step back and let her continue in this path that I know for certain s going to end bad for her?