With SharePoint 2013 the server part to edit/view Office documents with the browser is now a separate product and needs to be also installed separately. Therefore why not use this new server to show office documents from other sources?

We are using WOPI (Web Application Open Platform Interface) so that SharePoint 2013 and OWAS 2013 can communicate. A colleague wrote a great article what you need to know to implement WOPI into your own Web Application.

Title: Introducing WOPI
URL: http://blogs.msdn.com/b/officedevdocs/archive/2013/03/21/introducing-wopi.aspx