She wanted to laugh at that because if he knew who it was and what was going to happen to her, then he'd know that a safe word wouldn't do her any good. Also, at least she had made good on her statement of going after Granger. She may not have got him to do it but he was a prat anyhow.
"Goodbye, Zacharias. Have a nice evening with your mother," Marietta pulled out her wand and cast a curse to blow him back into the fireplace, tossing the Floo powder in simultaneously. She called out "Smith Estate" and shut down her Floo connection once he had gone through it. Now it was time to go get Granger ready.