What was the shipping weight from the tracking page or on the label?
Make sure Apple or someone files a claim with UPS. They should be able to track who stole it if it was the driver. My dad works for UPS and they have had this happen before with a T-Mobile phone that I can think of. If the driver did it that was incredibly stupid and he shouldn't have delivered a light box and should have said something about it.
I mean, unless it was a total fluke of the packaging but I find that really odd.
Sounds like your mom is enjoying her new iPhone!