Hello Lance,
Did your mom attached a signed letter in the email. Management agreements are basically real estate contracts and the proper way to terminate them is in writing with a signature of all parties that signed the management agreement. Just sending an email without a written signature is not necessarily an acceptable method of properly terminating a management agreement.
Usually management agreement can be terminated without any fees at the end of the lease agreement or any time the property is vacant. Again, I would recommend that you have a real estate attorney review the management agreement as they vary from company to company and state to state.
At this point I would have your attorney send them an official termination and to turn over all documentation and accounting for the funds in a timely.
I did a little googling and research on attorneys in the Memphis area and I found: Crislip, Philip and Associates, looks like they are familiar with landlord-tenant law - http://www.crislipphilip.com
The property manager should have given the tenant a receipt for each time the tenant paid cash.
There are some property management companies that give tenants a discount if the tenants pay on time or early. The property management agreement is a limited power of attorney and does give management companies the rights to negotiate rental amounts.
Also, I recommend you go after them for embezzlement if they do not provide you an accurate accounting of the funds. Judges take a very poor view of property managers that embezzle funds.
If the PM did not make any missing funds good you be able to get a judgment and submit it to the Real Estate Commission recovery fund.
In regards the lease agreement:
I know each state is different on their rules. In Texas, if you look at the parties listed on the lease agreement it is the owner and the tenant not the management company.
When you go to the signature page on the lease agreement that is where the management company signs on behalf of the owner.
I think the longer this situation goes, the messier it will become. My best advice is to be proactive as possible and engage a real estate attorney as quickly as possible, they will be able to you sound advice on what action you should take on your mom's behalf.
Hang in there!
Kevin