Hello! Thank you for your question today. My goal is to provide you with the information you seek.
I am very sorry you are dealing with this situation, not returning the keys is very ridiculous. The first thing you should consider doing is writing a letter certified return receipt requested demanding the return of the keys. By doing this you have evidence if needed that she received notice and refused.
If she still won't return the keys, an option, but the most expensive one is to file a action in court obtaining a court order whereby a judge orders the her to return the keys.
You could notify the clients and suggest they change their locks but this should be at your expense and then sue in small claims for the amount of money you are out of pocket, including loss revenue if you can't service the clients, as a result of the partners immature behavior.
Hire a local attorney to write a persuasive letter that it would be in her best interest to return the keys.
You may also contact the police and inquiry as to whether they would accept a stolen property report from you regarding the keys or if each client would need to file a separate report.
I apologize that this was probably not the answer you were hoping to receive. However, it would be unfair to you and unprofessional of me were I to provide you with anything less than truthful and honest information.
All my best & encouragement.
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All states have intricacies in their laws and any information given is simply information only and specifically is not intended to be, nor does it constitute, legal advice. This communication does not establish an attorney-client relationship with you.