My 18 year old daughter has been seeing this boy for about a year and has really fallen for him. He is a good kid and really tries hard but the problem is his parents.They are really abusive and controlling and on numerous occasions I have witnessed them not only being verbally abusive but also physically as well. He does nothing to provoke them and at times he shows up needing a place to stay. I don't want the kid sleeping in the street but when I let him stay his parents call all hours of the night, they are usually drunk and begin cussing not only their son but my daughter as well. The boy is 19. I have tried to get my daughter to break off the relationship because I feel she will be the one hurt in the end, but she refuses to. Any suggestions in this matter will be greatly appreciated I'm at my wits end.

They are of age to make their own decisions and mistakes, but you could encourage him to get a job and move away from the parents.

talaniman has a good point about the boy working and moving out. You said he's a good guy and your daughter deserves a good guy. Don't punish the boy for the sins of his parents. Be supportive by being an employment reference for him. Help him find an apartment. Help him get a restraining order from the parents and to get an unlisted phone number. If the kids love each other then he might need his future mother-in-law to be like a mom now. I know that he is not your responsibility but your daughter is. By pushing them apart you may draw your daughter further into the line of fire with the boy's parents. By helping him you will gain more of her trust. If you try to keep them apart she may move out with him and they may make many mistakes about security that could get them badly hurt. At least with you helping them they can have the benefit of your experience.
Who knows, you might get a terrific son-in-law from the effort that will love your daughter and care for her for the rest of her life. Worth the effort I'd say :')