Keep the original text message, take your mobile phone to the Police who will take action and at the very least issue her with a caution.
The same sort of thing thing happened to me when I was an operations manager with a training + development company, I had cause to sack of of my femail staff for gross misconduct, this girl sent both me and her team leader texts suggesting we were having a torrid affair, she was also saying that her team leader was perfoming sexual acts with me.
I informed the Police who took statements from myself and the girls team leader, visited the girl in question who admitted everything and issued her with a formal caution, they also said that if she sent any more of that sort of message again she would be charge formally, and placed on the sex offenders register
hope that helps