From the question I will proceed under the assumption your office manager is not bound by any contract, nor do you have in place any handbook that applies to her post-employment obligations.  With those presumptions in mind, if you are not offering any additional consideration (money/severance) there will not be a fair barter for her to sign anything.  However, you may want to try to have her sign off on a workforce confidentiality agreement or possibly a non-disparage.  

Happy to help draft.  Food for thought: next time lock up how she leaves before she starts.   

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