Years and years ago, when I was in my early 20's, I worked retail in a mall. About 8:50pm or so, two wealthy women in their 40's (judging by their clothes and jewelry), strolled into my small boutique store. They started in the front and looked over every piece of clothing in every rack as they moved towards the back. One of them eventually tried on a few outfits about 25 minutes after the store had officially closed (I shut the doors so no others could stroll in). Twenty minutes after that, one FINALLY looked at her watch and laughed, "Oh, Majorie! We closed out the mall again!" Ten minutes or so after that, (now five minutes until 10:00) they finally wandered out without buying anything. I still had an hour of closing to do, plus a walk through the deserted parking lot in the dark, and a 20 minute drive home.
I wish I could have said something to them along the lines of, "Sorry, ladies, the store is now closed" or what I really wanted to say, "The world doesn't revolve around you! Get the hell out you selfish *****", but I couldn't per store policy.
With regard to your Mom, I don't think it would hurt anything if she told the customer directly. "We're closing now! What can I help you with?" paired with generally making a nuisance of herself by stationing herself on the customer's heels so she can't shop in peace.