Did support ever get back to you? Holidays can make responses take longer of course.
What game did he get that he is having trouble with?
What is the exact error message?
Does he have any other Steam games and is he having a problem with them?
Some suggestions: Try on a different PC if its an option. Uninstall, reinstall. Make sure your DvD drivers, video drivers, sound drivers and any network drivers you have are all updated. Make sure the right ports are open in the firewall you use in your Operating System. If your anti-virus thinks it is suspect content, you can temporarily disable it for testing purposes, just be sure to re-enable after a success or fail.
It is entirely possible you were given a bogus seiral key from them. This has happened before with other game company's, in which case you have to wait until they issue you a new one.