She doesn't have to let him know it's an option! We saw Avon and Vila also fly the Liberator separately so I think the purpose was more to let Zen know their good intentions. Which I put into the story I'm about to post. :-) Anyway it seems that merely telling Zen their name and having them speak for voice recognition was enough, as in Power play I think it was?

Besides Tarrant might make Zen change his mind if he let Zen take a close look. He's still fairly Federation.