GMail rejects valid credentials in Outlook 2010.

Hello Experts:

One of my clients was using Outlook for his Gmail account.  It worked fine for several years.  Suddenly, it just stopped working.  The Gmail servers began to reject his credentials.  I logged onto his account, using the credentials he gave me and they worked.  But those same credentials fail in Outlook 2010.    I checked and double checked all the settings and found everything in order.  
I repaired Outlook; I verified that POP was enabled on his PC.  All the settings are correct (I've checked and double-checked them).  

I’ve run out of ideas.  Does anyone have any suggestions?

Thanks

thedslguy