Steps to reproduce :

1) As an administrator create a community called test
2) Change the friendly url of the test community to /test1 (it will be /test by default)
3) Add a document library portlet to the test community
4) Upload a document called Doc1.doc
5) Go to the file details and copy and paste the webdav url in another browser window (eg. : http://localhost:8080/tunnel-web/secure/webdav/test1/document_library/Doc1.doc) You should be able to download this file
6) As an administrator create another group called test1
7) Open the webdav url from step 5 again and try to download the document. You will not be able to

The problem is depending on your viewpoint in the getGroupId method of com.liferay.portal.kernel.webdav.WebDAVUtil or in view_file_entry.jsp

Basically if the url is http://localhost:8080/tunnel-web/secure/webdav/test1/...

It will first check if test1 is a groupname. If this is the case it will look for the file in that group. Only when it finds no group that has a matching name will it look at the friendly url

Unfortunately view_file_entry.jsp uses the friendly url path in the url, never the name