Hi,
It all depends on the terms and conditions you set out when you employed them.
Myself, I give them a copy of said terms, With a clear date of how long probation is. They also receive a copy of my staff handbook which outlines how they will be dealt with if standards are not met ( This is my disciplinary procedure ). Finally I ensure that all training logs are signed to prove that they have had the training and are happy and except they know how to do the job correctly.
In your case, It might be best to wipe the slate clean ( Pardon the pun ). If you have covered yourself legally with your terms and she is still in the probation period, Then you can let go.
In my terms I use this,
On joining ( My company name ) you will be required to serve a 3 / 6 month probationary period. During this time you will be thinking about weather you wish to remain working with us. Your line manager, at the same time will be assessing your suitability for your role. If you do not reach the standards required by ( My company name ) your employment may be terminated at any time during the probationary period with one week's notice. ( My company name ) reserves the right to extend the probationary period where required.
The ACAS code on Discipline, Dismissal and Grievance Procedures do not apply during the probationary period.
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That above is taken from my handbook but in the copy of terms it gives a brief outline but refers to the handbook for full details. I ensure that I have signed copy's of both terms and the receipt of the handbook plus full acceptance of the handbook terms and a understanding of them.
In the end you need to cover yourself. So if you have you can let go or best option is like I said, Start fresh with her but make sure your fully protected and everything is signed and legal.