It may be related to his permission on the document library or documents in question. SharePoint has 2 read only type permission levels, one called "Read" and another called "View Only". The main difference (that I'm aware of) between them is that "View
Only" will try to only allow users open documents through the browser, or to quote it's description in SharePoint
"Members of this group can view pages, list items, and documents. If the document has a server-side file handler available, they can only view the document using the server-side file handler."
I'd suggest checking if his permission level is set to "View Only" and to try changing it to "Read".
The only other thing I can think of would be to check the library settings, under advanced settings there is an option for whether documents should be opened in the client or on the server. But this should effect all users the same.
