Try replacing the cap on the expansion tank with a new one.  Some times these
go bad and cause problems.  If this is not the problem then the only other
thing I can think of is that somehow, someone has switched the hoses at the
radiator (like if they were flushing out the system and then didn't remember
which pipe was which).  Ask the customer exactly when this started happening
to them and if they recently either flushed the coolant or had it flushed.
Hope this helps.

Thanks,
Ken Wilford
Van-Again
John 3:16
http://www.vanagain.com
Office: (856)-765-1583
Fax: (856)-327-2242