Ask an Immigration Solicitor. Get an Answer ASAP.
What do you want to achieve by reporting her to the police?
What you can do is go to the police and tell them that you are aware that someone who used to work for you but is no longer working for you is an illegal immigrant and that you know their identity and where they’re working provided they will confirm that you will not be prosecuted for previously employing someone who you no longer employ.
Can I clarify anything for you?
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We can still exchange emails. Best wishes. FES.