In Plus None we find out that Nico didn't really kill her boyfriend, she is just trying to extort money from Wilhelmina, with her boyfriend posing as a private investigator blackmailing the Slaters. In desperation, Wilhelmina planned to flee the country with Nico, and resigned from her position at Mode effective after the upcoming ``Fearless shoot'' in the Bahamas. However, Marc later overheard Nico and her boyfriend scheming, and after initially refusing to believe him, Wilhelmina set a trap for Nico by showing her an expensive family heirloom that they could use to pay off the blackmailer.
Marc
Nico