Well, she'd have to figure out what happened with the broken Stargate while the Doctor tried to figure out what happened with the TARDIS not working-- they'd have to cooperate to get back, and in the meanwhile (especially given the Doctor's love for trial-and-error) might end up other places than they meant to go-- like back to Earth!

Or it could be something cataclysmic: if she doesn't go with the Doctor and help him, something horrible would happen, oh, like say the destruction of Earth or an invasion of Daleks through a Stargate, or some other disaster that he would need her help to prevent...