She wasn't stupid, she was naive and sheltered. She grew up as a sheltered Baron's daughter who didn't seem to do too hot socializing in high society. I don't think she ever got that talk every girl should get of "just because he acts nice doesn't mean he is, sometimes men might want something special from you...don't give it to them."
Her dad wasn't exactly the best dad and her sister looked down on her (probably due to her own frail self esteem). She didn't know any better. There's no internet in this age to tell you that Alan gave off giant red flags. Even in that internet age, there's still innocent girls that get fooled by predatory men.
Alan not only gave Daisy an opportunity to make a living to support her sister and herself, but he also treated her well. For somebody in dire straits and with low self esteem, he must have seemed like an angel. She probably thought hers and Jasmine's future would be settled (with marriage). The blame is entirely on Alan and his homicidal fiancee (or whoever ordered it). I wonder how many women have been murdered that way?
Given Alan's position as Daisy's boss, I wouldn't be surprised if he purposefully targeted her because she was somebody who couldn't say no. Even if she knew better, she might not have been able to. His reaction to her wanting to break up was telling. He didn't just say "you're nothing without me", he actually hit her. He didn't just cheat, he took advantage of a disadvantaged and naive girl. Maybe that was what he intended from the start. Then he had the gall to hit her. That's abuse.
There's men who will target women with low self esteem for sex and it's usually serial, which is to say that even if it ends badly, they'll just pick up and move on. They don't seem to understand the problem is with them, not the women they target.
We really can't blame poor Daisy for this one.