I would suggest that you call her back in when you have finalised the investigation and advise her to have a support person.
You can choose to give her a warning letter and also advise her that she will be placed on a management plan to make sure nothing like this happens again.
You have to also set out what your findings were and based on her admission she has harassed the Manager and that is unacceptable.
You also have to speak to the Manager and again go through with the policy with him just in case so she cannot argue that you have favoured him.
The following link is the small business dismissal code from Fair Work. Look at this and make sure you have complied.
The closer you follow it the better if she is going to claim.
Good luck with it, it is not an easy job to do but you have to be firm with both and make sure that they have been given all opportunities to change. The threat is an issue for her and it can be dismissible offence. But you have to do thinghs correctly.
I hope this makes sense and is of assistance. If there is nothing further
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