Should I be okay to upgrade to DevEx 6.2, or will this cause problems with the inherited controls since they are compiled with 6.1.5?

Yep, you were right.  We uninstalled 6.1.5 from his machine and everything magically worked.  The weird part is that I have both versions on my machine but don't have problems.  I figured it's something to do with me building it on my machines that is allowing me to avoid the issue.  Oh well, it's fixed...I should've asked 3 hours ago 

.NET resolved some of the "DLL hell" that you had with the old Win32 stuff, but it introduced a whole new group of problems