Becareful!! If she didn't leave on her last day, the resignation letter will be void, so once your new one arrive, she will claim you terminate her and ask you for one month payment in lieu. Furthermore, if her old employer want to hire her again, she has to get a release letter from you in order immigration will process her new application, without that she won't be able to apply a new job.
If her contract will be finish soon, then she can go to immigration to declare, but she hasn't finish her contract, she has to have a prove to immigration!! ?-(