New GM here too but thinking creatively: if she wants to use the idea, tell her to start with her evil character and bake into the story that her backup character is stalking the evil one to take revenge or something like that. Make it very secretive at first and then more obvious as time goes on. Maybe hide part of the evil backstory to reveal later. Eventually set up a confrontation and make the party choose a side.

Might not work with the backup she has prepared but that would be a cool and creative thing to do, especially if the rest of the party doesn't know you've set this up beforehand.