...Well, I mean, what did you expect?
Everyone here runs on impulse and emotion, basically, so of course he'd trust the guy that's been serving (and stealing from) him for years over the woman he keeps finding things to blame her for.
That said, there should be some natural consequences for this, and I'm surprised anyone is still considering him the male lead at this point.
For me, the best result is for her to pave her own way to happiness, for him to come around and want to connect with her, and for her to say no with a smile.