Here is the text version of the error he was getting:
Unable to display this Web Part. To troubleshoot the problem, open this Web page in a Microsoft SharePoint Foundation-compatible HTML editor such as Microsoft SharePoint Designer. If the problem persists, contact your Web server administrator.
I logged into his site, but it initially rendered just fine for me so I figured it was on his end, but the next day, I too received the error. So I looked into the ULS Logs on the WFEs to see if I could identify the problem. As with most cases, the details were limited but there was one thing that stood out that I was able to search on:
Error while executing web part: System.NullReferenceException: Object reference not set to an instance of an object.
After a quick search, I was able to track this down to a Microsoft Security Update that was applied to our web front end servers over the course of the week. We have two load-balanced Web Front End servers in our farm and it turns out that both did indeed receive the automatic update. One server updated on the 13th of July (this was the server that the end-user was hitting) and the other server was updated on the 17th of July (which is the one that I was hitting and the reason why, a day later, I was getting the error).
At this point, I needed to check the OS to find the exact KB update that was associated with the issue.
Windows XP and Windows Server 2003: KB2844285
Windows 7 Service Pack 1 and Windows Server 2008 R2 Service Pack: KB2844286
Windows Vista Service Pack 2 and Windows Server 2008 Service Pack 2: KB2844287
Here is the Microsoft Security Bulletin for July 2013:
Microsoft Security Bulletin MS13-052
As a temporary workaround, we found uninstalling the update (KB2844286) from the servers (all WFE’s and App servers), rebooting, and then performing an IISReset on the servers will resolve the issue. Several others have experienced this issue as I found here: http://social.technet.microsoft.com/Forums/sharepoint/en-US/cc9a557b-93cd-40d5-965c-e0a2f107624d/unable-to-display-this-web-part-error-message-after-patch-kb2844286 and here: http://social.msdn.microsoft.com/Forums/en-US/871a0661-05d0-4f3a-b66d-2504552142bd/error-while-executing-web-part-systemnullreferenceexception-object-reference-not-set-to-an
At this point, this is the best information I was able to find.
Also according to other blog/forum posts I've read, it appears this error "typically" appears when a list or web part view has had the XSLT customized.