As a minimum they should get permission to move their cars right next to the entrance while the guard is still around so they do not have to walk far when he is not around any more.
Would it be possible to have the guard stay another hour?
Good idea about moving the cars.
My understanding is that the unarmed but uniformed guard is mostly to deter serious shoplifters. I also expect that the contract with the security company is for a certain number of hours each week. Paying the guard to hang out for an unpredictalbe number of extra hours (straightening up the store can take a several hours at high-traffic times of the year) probably would be a no-go, but again its worth asking about.
Thanks for all the ideas!
https://www.ojjdp.gov/ojstatbb/offenders/qa03401.asp?qaDate=2010 suggests that the risk of crime varies by time of day, so the before and after hours risk of crime may not be the same.
Spoke with MIL today, passed on some of the queries suggested here, and learned more about the project. The store manager is the one who wants this letter written, and will sign it along with the employees to pass up the chain of command. I got volunteered because I’m an over-educated native speaker/writer. The letter isn’t a translation. It is to be composed in English with the information that they want included. I will draft it, the manager will read it and edit for content, and I can fix the grammar. Whether together we can hit appropriate business-style remains to be seen. 
Oh good that no translations are involved!
“Whether together we can hit appropriate business-style remains to be seen.”
I don’t think you need to worry. You’re a good writer. And google is your friend for learning how to format business documents.
You just need to be sure that the letter reads as a respectful request from a manager who is concerned about the safety of his/her valuable employees (with some suggested solutions).