Where is he getting the DAT from? NaiHTTP or NaiFTP?
You should check its agent log to see if he cannot contact your ePO server
for example your sitelist may contain a repository that only some of your clients should ever use (clients in a remote office) others not . Depending on the sitelist processing option you have set (Use this repository order, Select by ping time, Select by IP distance, etc.) other clients might use that "forbidden" repository unless you prepare a different sitelist for the other clients where you disable that repository.
Please also take a look at "Disable new distributed repositories by default" checkbox (ePO 4.5) or "Automatically allow clients to use newly added repositories" (ePO 4.6) checkbox, whith which you can prevent clients from using new repositories automatically.
I hope I could be of any help.