In fact, Emily did have a relationship aside from friendship with Ethan. They were having an affair. Emily's husband suspected her of cheating, so he hired a private investigator to find out. Ironically, he went to Ethan Grossman's agency. In his cruel sense of humor, Ethan waited one week to tell Emily's husband about her affair, and even sent him a bill for his "hard work" on the case. That is the reason Emily's husband divorced her.
Before you go and accuse her though, an interesting point is raised in Ethan's will. In the event of his death, he leaves his entire detective agency, his current cases, and all of his assets to the Manuel, Rick, and Emily. HOWEVER, he also says that they are only entitled to everything if they can figure out who killed him. It would seem that Ethan knew he was going to be killed before the guests had even arrived.