Yes, he should set some boundaries even though she is 18. From what you've shared she is not very self sufficient.
The issue here is that he had let her do as she wants all of this time. It would be difficult to restart parenting of someone who is 18 and had not learned certain behaviors before then.
Of course, if he is financially supporting her, then she ought to at least have the courtesy of being appropriate in the home. This would be something that he and her would need to work on (involving her boyfriend and his parents can be secondary)
You've shared that you've already spoken to him. His reaction would depend on what he is willing to do. He may not want to be bothered with parenting issues with all of the other stuff going on. His behavior may be just adopting the path of least resistance. He may be doing more damage to this young lady in the long run though. She would need to learn respect, boundaries and responsibilities and self reliance.
Perhaps if you approach the situation from that stand point he can see your concern and logic behind it.