Hey Matt,
First make sure you're using the latest version of TurboActivate (currently 3.4.3).
Today he got a "zero days remaining" when he started the app - and subsequently uninstalled and reinstalled a number of times (not on my advice).
It sounds like he might've "played" with the time on his computer (rolling it back to get extra free days). TurboActivate detects fraud like this and says "nope, I'm setting your trial days to 0".
In an effort to find out what was going on, I sent him a trial extension - he gets the error "you must enter a valid key" - the key is valid (assuming his cut and paste skills are up to the job).
He probably entered it into the "product key entry box" (instead of the trial extension entry textbox). They're different keys. The product key entry box would rightfully say "you must enter a valid key" because trial extensions aren't product keys. If you're using the TurboActivate Wizard, tell him to go to Other Options -> Extend Trial and then enter the trial extension there.
If he's still getting an error, even after telling him how to enter trial extensions into the correct box, then tell him to take a screenshot of the screen and send it to you. I'm sure he's entering it into the wrong screen.