Auto updating product quantity via email

I have a friend that is using Zen Cart and he was asking me about an issue he's having.

He's running a site that uses Zen Cart.
The problem is that when a customer purchases a product, he then goes to his distributor to place the order and sometimes they are already sold out. He discussed this with the distributor and they are now sending him these emails that have urls to .xml and tar files that contain updated quantity information for all the products he sells. Is there a way to incorporate those emails into Zen Cart so his inventory quantity would reflect the updated information?

The emails come in every 30 minutes or so, i'm no webmaster and have no idea how to connect the xml files to Zen Cart.

Any help would be greatly appreciated.
Thanks

The xml's are in the suppliers DB format and need to be converted to a ZenCart DB format

How is that done?

There is no built-in functionality to automagically import unknown-format xml files into Zen Cart.
You'll have to write your own custom code to read your suppliers' unique format and stuff the data into the database after studying how the data structure works in Zen Cart.